Reject an offer from Oxford for another uni?

I would note that Warwick maths (even diluted in a joint honours degree) is also going to be fairly intensive afaik, particularly first year. But I'm not sure it's necessarily less pressured, just perhaps in a somewhat different way (I got the sense mathmos at Warwick have to fend for themselves a bit). @_gcx might be able to offer some insight into maths at Warwick in that frame? Bear in mind Warwick maths is generally considered comparable if not equivalent to Oxford (or Cambridge) maths (albeit you are considering between Oxford physics and Warwick maths & physics, but relative "quality" of Oxford physics is likely to be similar to Oxford maths). So even if you do go to Warwick, you aren't necessarily "going down" in quality so to speak.

In terms of the courses, the question is, do you want to study the kind of maths that makes up a maths degree? Bear in mind you will study maths in a physics degree, and a lot of it! You'll be using mathematical methods every day, and I expect on a course like at Oxford you might be more likely to spend a bit more time on more abstract formulations of those methods and covering some more applied forms of what is fundamentally pure maths (e.g. group theory).

Also bear in mind the different style of teaching for each, as at Oxford you will have the supervision format, having a supervision with an academic once a week with 2-4 other students. This does then offer the opportunity for some very close engagement with academics, although does also require you be able to communicate your thoughts on and reasoning for a problem in that environment (which I definitely can see would be a stressful environment for some - some people do better where they can fade into a large lecture hall!).

The difference then is on a course like at Warwick you will probably spend a reasonable amount of your time doing maths also doing more proof-based pure maths, alongside the necessary methods-y topics needed to support your physics material. So if the choice of maths and physics at Warwick is more out of a belief that you would do less maths (or less frequently) in a physics degree, then that is not really accurate and either course would suffice. If it is specifically because you do want to write analysis and algebra proofs alongside doing physics work, then of course the Warwick course may be more appealing.

You really do need to disregard your boyfriend as a factor here, imo. Not to be overly negative but, the odds of a school couple still being together by the end of a 3-4 year degree are honestly quite low. If anything going to the same uni might just make things more awkward if (or perhaps, when) you break up. It would be especially bad for you if you did break up, and on reflection you decided actually you would have rather gone to Oxford but picked Warwick to be with him and then regret that decision and "what could have been".

There are reasons to choose Warwick over Oxford, but you need to be doing so for the right reasons; fully understanding the differences in the nature of teaching, the content and style of the courses (particularly the extent of maths done in a physics degree, and the type of math's done on the maths side of a joint honours degree), and not on the basis of interpersonal relationships which may or may not last for your degree (much less the rest of your life). Taking into account your mental health and how that may (or may not) suffer at Oxford (and whether this may happen at Warwick as well even) is important too.

I think a lot of people say 'never reject Oxbridge' because they know how hard it is to get in, and a lot of people would even be jealous of the opportunity to study there. I think choosing ANY university isn't a decision you should make quickly, and it's no different here. You should consider what the initial reasons were for you to apply to these universities, and whether these things have changed. Definitely go to the virtual open days and watch youtube videos of the universities/ students at the universities to try and get the best possible picture of what it will be like to go there. Then make a list of pros and cons for each uni to try and think things through a bit clearer? (could also be useful to explain to your family why you feel the way that you do if you have a clear list)

I think Oxford could give you some amazing opportunities and will definitely give you a good education, but Warwick is also a great university too. You've got to consider things which are more than just your education also, like your social life and the differences in the courses. If you are concerned for your mental health, I've heard that Oxford can be quite heavy on the workload but I don't think it's significantly worse compared to many other universities, its just that people talk about Oxford being strenuous and academic because it is the stereotype. What are the things that you dislike about Oxford right now?

You should feel really proud of yourself for getting this far with it and if it's really not for you (a good way to measure this is if you feel like you're actively looking for ways to dislike Oxford), then you should choose Warwick. Don't take all of your 6th form head's advice on this because unfortunately he will want you to go to Oxford because it sounds good for your school. Your family and boyfriend probably want you to think it through more because they've heard of the stereotypes of Oxford being so competitive and elite (which is partly true!), but at the end of the day you know the course at Oxford & you're own personal preferences WAY more than they will. And you're the one who will have to spend the next 3 years there. The only thing I will say is try not to be persuaded by where your boyfriend is going- you'll make it work in either scenario if you want to be together. I can't tell you with certainty either one to go to since I'm not you, but I wish you the best of luck wherever you go! They're both brilliant unis so congratulations on the offers -you've got until May (I think) to decide too so don't stress it all right this second, you've got plenty of time to think things through.

Actually, someone did something close to this (rejected Oxford maths for Warwick maths). Unfortunately, I don't know them well enough to put you in contact with them.

Oxford maths to Warwick maths wouldn't be considered a step down (indeed some would say it's a step up). The main advantage in going to Oxford in terms of the maths would be all of the Oxbridge-isms, but those don't seem particularly important to you. (they certainly shouldn't take precedence over the course content) I'm not sure about the quality of the physics department though. Definitely not quite as highly regarded but still quite good.

I do straight maths at Warwick as @artful_lounger said so I'm happy to answer any questions.

It's good to know that there are people out there who've done that and I'm not weird for wanting to turn it down XD
Would you mind explaining what you mean by 'Oxbridge-isms?' I don't think that's a term I've come across before.
Thank you for your help!

Just the stuff kind of unique to Oxbridge - the whole vibe, formals, supervisions, the college structure, (I don't think colleges in other universities are autonomous to the same extent) so on.

This feels like really helpful advice, thank you! Honestly I think the main reason I applied to Oxford was because I had friends applying and I'm quite competitive so I wanted to prove to myself that I could do just as well as them. I don't think I ever had much of a reason to apply other than that, and so now you've mentioned it I do think all of the research Ive been doing so far has been to try and make myself dislike Oxford enough to not feel guilty turning it down, and looking at some other state school educated people's experiences there have kinda been successful at that, but at the end of the day I feel like I don't have much of a choice.