Go for either UCL or Warwick! They're both such strong universities!
The maths course is much more intense at Warwick, however, if you're good at Maths, then you'll get the best degree anywhere.
In terms of academia, Warwick is stronger. In terms of worldwide rep, UCL- being in London, is obviously stronger.
UCL and Warwick are both so very highly reputable in the UK. I think you should take advice from only students from top end unis, not from anyone else because they can give you the best advice. You can't miss an offer like that from Warwick for mathematically related subjects!
I'm doing Maths at Imperial, Warwick was my back up even though the offer was the same hehe, just to let you know, so I'd choose Warwick. However, if I was doing something like Law or Sociology, I'd choose UCL as they are much better for the arts degrees.
To be quite frank, they're both amazing, so what ever you decide to choose, you've got MONEEEYYYY XD
good luck x