How has your local area changed due to immigration?

Right, but you have attributed the relatively higher level of littering within immigrant communities to that normalisation. This isn't necessarily the case. Normalisation (within their country of origin) could play no role in how much littering they do within the UK.

What makes you say that? If a community in another country has a high inclination / normalisation towards littering, why would they suddenly lose that as soon as they enter the UK?
What else could explain the fact that they still have a higher level of inclination / normalisation towards littering after entering the UK, if it’s not carried over?

If the reasons a person litters in country X changes when they move to country Y (e.g. their income, education, etc) then there's no reason to believe that the amount they litter will be the same in both countries.

A lack of waste management procedures would only explain why litter isn’t being cleaned up effectively. It wouldn’t explain why more litter is being dropped in the first place.
At the park and the cricket stadium I volunteered at, there would consistently be far more litter after events that were targeted towards and popular with these immigrant communities. There was no problem with a lack of clean-up; people like me would collect all the litter by the end of the day either way. There was no significant difference in environment; it’s not as if we’d remove all the bins during events that immigrants were attending but keep them otherwise.
There’s no way to explain this without factoring in the behaviour of the people in attendance: more litter was dropped at these events because more people were more willing to drop litter in greater amounts. Ultimately, for litter to get there in the first place, someone has to be willing to drop it. So it can’t possibly be disassociated from people’s behavioural inclinations.
But the point is that those things don’t change when a person moves country; at least not immediately. If a person lacks education they’re not going to suddenly become educated as soon as they step off the plane. If a person has been raised in poverty then that’s still going to be true after they’ve arrived. These things take time to change, especially on a macro level.

If waste management doesn't exist then it is very easy for litter to build up, even if people make an effort to control their rubbish and do not actively litter (at least any more / less than people from another country). What I am referring to here is why litter may be a problem in one country but not another, not why people litter in your city.

Again you are not arguing that immigrants from your city have merely an inclination to litter, rather that they have a significantly higher inclination to litter than British people would have (all else being equal, e.g. in poverty, education, etc). You've attributed this to a difference in cultural attitudes, but you've ignored the possibility that all else is not equal.

In other words, you've not done anything to show that these streets / communities would have less litter if they were instead populated by British people (with the same wealth, education, etc). You've mentioned the example of festivals but you've not said whether all else is equal. Are the people who attend non-immigrant festivals the same in all other regards than those who attend immigrant ones?

Again, that may explain why you see more litter on the streets in one country than another, but it wouldn’t explain why one country’s inhabitants are actually dropping more litter than those of another. For example, Japan has hardly any public bins or public waste management expenditure, because people are expected to take their litter home with them. And you still don’t see litter on their streets, because people actually litter less (as they’re taking litter home with them).
I don’t see why we need to consider “all else being equal”. The whole point of being an immigrant is that all else is not equal; you’ve been raised in a different country with different levels of poverty, education, and a different environment more broadly.
I’m sure if a White British person was raised in precisely the same way as an immigrant under precisely the same economic and educational circumstances etc. they’d learn all the same behaviours and be just as inclined to litter. But the point is that they haven’t been raised in those circumstances, generally speaking, so they’re less inclined to litter.
If I were claiming that British people were somehow innately less inclined to litter and it had nothing to do with their environment or circumstances, then yes I’d need to show that all else is equal within those environments and circumstances. But that’s not what I’m claiming.

If you wish to blame immigrants for ruining your city on the basis of them not following British culture then, yes, you would need to show that all else is equal. If the amount of litter in these communities would be the same if British people were living there then the issue isn't immigration or non-following of British culture.

But I’m not blaming it on immigration in and of itself. I’m attributing it to a very specific pattern of immigration.
Poverty and education etc. may well be the underlying reasons behind the difference in behaviour. But this is a pattern of immigration that is particularly bringing in people from more impoverished and less educated societies, and not providing much reason to adjust their behaviour when they get here. That is, they are more affected by the factors that result in someone having the inclination to litter.
So it doesn’t make sense to require that “all else is equal” because this specific pattern of immigration inherently involves those things not being equal.
It’s a bit like me saying “John can afford a Ferrari because he’s a CEO” and you saying “But if all else were equal (e.g. income), Jim the bus driver would also be able to afford one”. Yes, you’re right, but my point is that the difference in profession is the reason why there’s a difference in income, and hence a difference in ability to afford a Ferrari.

If you accept that poverty and education are the underlying reasons behind the increase in litter then why are you blaming the immigrants for not following British culture / not having British values? The issue wouldn't be cultural, it would be economic. After all, if these communities were populated by British people with comparable levels of poverty and education then they'd be filled with the same amount of litter.

Because an economic issue is a cultural issue. A difference in the economic environment of a community is one of many systemic causes of differences in the usual behaviour and values of people within that community. And that’s exactly how I’ve defined a “cultural difference”.
Part of the reason why British culture and British values are what they are is because of the relative wealth that the country has enjoyed, on the whole. So I don’t agree with the logic that “it’s economic therefore it’s not cultural”; they’re part of the same package.
Refer back to the example of the CEO who can afford a Ferrari. You could ask why am I attributing it to his profession rather than his income. But in reality I’m attributing it to both, because again, they’re part of the same package.

Why go out of your way to define this as a cultural issue when it is clearly just an economic one? Culture is about the ideas, customs and behaviours of a group of people. It transcends economics, it isn't just about what people do in their life. You can have people on opposite sides of the economic spectrum who don't litter, you can even have people on opposite sides who do.
Even if you did want to define culture in this way then what makes non-littering part of "British" culture? Plenty of British people engage in littering, if anything littering is part of "British" culture.

That's exactly the point I'm making. I don't think it is "just" an economic issue, because that would imply that most poor people litter, most rich people don't litter, poor people stop littering as soon as they become rich and rich people start littering as soon as they become poor. I don't think the relationship is as direct and individualised as that.
The relationship I see is more on a wider population level, where a poorer society is more likely to have more people who litter more, on average. (And of course economics isn't necessarily the only underlying factor either; education, acclimatisation to seeing litter and all kinds of other features of the societal environment could be playing their part). In other words I'm talking exactly about the resulting ideas and behaviours of collective groups of people rather than individuals.

It's not the point you're making if you are trying to frame economic issues as something cultural, unless you're deliberately using an extremely abstract definition of the term "culture". If the issue is economic then immigrants nor culture are your problem, poverty and education are.

If you wish to define culture in such a broad way then that's up to you, but in that case you've not explained why littering isn't part of "British" culture. How do you define what is / isn't part of a culture? Plenty of people litter, it's not like it's an uncommon thing for people to do.

I’ve already provided my definition of “culture”. It’s just the set of normalised behaviours and values of a group of people. (You gave a very similar definition yourself). That means if it is more normalised for one group of people to litter than another (for whatever reason), then that is by definition a cultural difference.
The issue can be indirectly economic as well, given that the economic environment is one of many possible influences on a culture. But I disagree that it can be just economic; otherwise we’d be seeing all poor people littering and no rich people doing it.
It’s all relative, isn’t it? Some cultures are extremely anti-litter, like Japan and Singapore, some a bit less so, like Britain, and some places significantly less so, like the towns and cities these particular immigrant communities have come from. It just depends on what we’re comparing against.

If you wish to define culture in this way then that's up to you, but you've not shown that these immigrants have been littering any more than British people would. If the level of littering they engage in is roughly the same as the sort of littering some British communities engage in then it wouldn't be a cultural difference.

I've concluded that it's likely that they have been littering more so than British people would, based on four pieces of evidence:
Obviously nobody can prove beyond all doubt that the average British community wouldn't be littering just as much if they lived in the same areas today without conducting some sort of gigantic scale experiment. But to say that these immigrant communities have brought with them a higher inclination to litter than the general British population seems to me to be the simplest way to explain all of these observations in one go (i.e. requiring the fewest additional assumptions, as per Occam's Razor).
By the way, if a specific sub-community of British-origin people were hypothetically littering and engaging in other antisocial behaviour in exactly the same way I would still say that they too are exhibiting behaviour that is at odds with British culture and values. But in that case I'd simply clarify that I mean mainstream British culture and values. Because of course, their culture still originated in Britain so it can't be attributed to anywhere else; it would just be an anomalous subset of British culture.

I don't see how your explanation requires the fewest amount of assumptions, it seems to depend on plenty of them. General economic decline and weaker public services are a far simpler explanations for what you've observed, plus it would match what can be seen in other cities.

Do you think British communities that litter the same amount don't exist?

It’s a simple explanation but I don’t think it explains all four of the things I pointed out in one go. It seems like a plausible explanation for point 3 (more littering over time) but it doesn’t account for the consistent correlation between littering and immigrant status in all four of those observations.
Whereas the idea that these particular immigrant communities have a higher overall inclination to litter (for whatever reason) explains all four observations. I’m not sure what additional assumptions you think are being relied upon with this explanation.
I don’t know. They might do, but I haven’t seen them.
Note: In recent posts I’ve been saying “littering” for simplicity but I’m actually referring to a whole range of anti-social behaviours as described in my original post.
I’ll also point out that there are some anti-social behaviours that I think these immigrant communities are less inclined towards than the general British population too - for example those related to alcohol. So there are some positives as well.

You also require assumptions about where these immigrants have come from and how normalised littering is in their country of origin.