I've been to both Warwick and UCL on many occasions and personally from what you've mentioned I'd recommend going to Warwick. The uni itself is a campus uni so you are essentially only surrounded by students and the people working at Warwick which is quite different from UCL in which London becomes your campus in a way. Having lived in London, it makes you feel quite small and that can feel isolating but only if you're out and about by yourself. If you're with friends you do not feel isolated at all and London becomes a bit of a playground so honestly it's a matter of perspective. London however, isn't as expensive as you think especially for students as there are student discounts everywhere and I've noticed accommodation (halls) in London for students is only slightly more expensive than in other places. In terms of Warwick, Coventry is only a 10 minute bus ride from the uni itself and there is so much to do as well so you definitely won't feel bored. Coventry Uni is also there meaning Coventry feels a bit like a student town as there are so many students and young people. Also, if you like the course more at Warwick id recommend going there as uni work can get tough during 2nd and 3rd year and you'd rather be doing something you like.