Well at any prestigious university there are the rich international students (tuition fees for them are £20-30k or even 50-60k for some stem courses), and being in London UCL has a high intake of those. Think flashy designer labels, ice latte and macbook, expensive restaurants and takeaways. But they usually just stick with each other anyway so I doubt it would make a massive difference.
And yes, there are the ex-boarding school / London private school kids who appear less flashy but also somehow seem to know everyone and have been to the right places. But actually the ones who go to UCL usually chose it for the liberal, diverse and metropolitan environment that it stands for, so from my experience they're quite down to earth and generally less stuck up than e.g. Durham, Oxbridge, St Andrews, Bath, Exeter...
Also - and this is the main thing I wanna say - most of the people you'll meet will be neither of the above, just your regular home students from all sorts of backgrounds and all over the UK - especially if you opt for the cheaper, self-catered halls for accommodation. Unlike many universities, UCL is apparently pretty good with putting in your 1st or 2nd choice!
If your heart is calling for Bristol honestly go for it, but imho it seems like a waste to turn down UCL otherwise xx