I just graduated from UoB this year, so hopefully I can give you some insight into some of the things I wish I knew before starting.
Accommodation: I was really worried about this one. I'll be honest with you, It really is a bit of a gamble. However, the gamble was more so to do with 'who' you get placed with, rather than the actual accommodation flat itself. You could either love your flatmates or you could hate them. I'd say, after talking with quite a few people on the topic, you are most likely to feel indifferent towards most of them (but there's usually at least one or two people you should like enough to chat with regularly)
The accommodation standard is more to do with your budget. I'd recommend looking at the reputation of each hall when deciding, and deciding on your priorities: catering options, location, price, 'vibe' etc. Then choose somewhere that aligns with this. But please don't get disheartened if you don't get your first or second choice, there are pros and cons to most of the accommodation. I'd personally go Clifton or Stoke Bishop (somewhere you can get to and from easily with a lot of other people close by) but again, that's just my preference. I got into University Hall, it wasn't even my first choice, and I was gutted. But it turned out pretty well, and I saved so much cash.
What I wish I knew before I started though, is that not getting along with my flatmates is not the end of the world. I would have told myself that I would make a lot more friends from interest-based societies and my course, than from my accommodation. And that there are so many ways to make friends at Uni, you just need to put yourself out there (and leave your ego at the door).
Study: Okay, so this might be different for everyone again. But I'd tell myself that getting a first-class degree is really hard. I didn't study that much in my first year. And, as far as I know, most people don't. You will be fine if you don't work super hard, and will most likely end up doing okay, (2.1-2.2) but if you want a first-class degree, I wouldn't slack during 1st year.
Social: There is a huge drinking culture at Uni. I think most people are aware of this. Drugs are also commonplace. You most probably won't be forced to drink or do drugs unless you get in with the wrong crowd and people are generally super chill if you don't want to drink. However, I will say, some societies base most of their socials on drinking, and if you don't do this, you may feel left out. Again, If this is you, there are so many other options available that won't put you in this boat. If you do end up getting into the drinking culture, with is completely fine too, I'd say be aware of the impact this can have on your mental health and your ability to perform well in your studies.
Finance: Bristol can be expensive, but not ridiculous if you sort out your priorities before you go. Do you like takeaways or home cooking? Do you want a nice room with an en suite or are you out all the time and just need a bed and a shared bathroom? Would you prefer living in a good location versus having a nice big house/flat? Do you prefer to Study at home or at Uni?
You'll likely have to compromise (unless you're super rich) Personally, I'd decide what you really want and then take a hit on the less important things. I had to get a job at Uni which actually helped me keep a good routine, and gave me some extra cash to do the things I enjoyed without feeling so limited all the time.
This is all that I can think of at the moment, if you have any more questions for me let me know, and I'll be as honest as I can!