Can I ask why you applied to Cambridge if you didn't like the course or the uni? (It's fine if you only applied because you felt pressured/just wanted to see if you got in - it's the same for me).
I'm kind of in the same boat as you, but with Oxford and a different course - my main deciding factors for me were the fact that I don't think I can really afford to live in London, and I also love Oxford as a place. Have you considered the financial aspects?
Also, are you planning to undertake internships/work experience during your degree? UCL will have very good career based contacts, and living in London means that you are close to these opportunities. I don't know how much time you'll have available for this, but maybe get in contact with a current UCL student? Basically, if you can get good work experience during your study at UCL, this can potentially outweigh the benefits of studying at Cambridge.
Remember that studying somewhere you really don't like (and somewhere that is likely to be quite intense and stressful) can really demotivate you and lead to a lower degree. Maybe consider whether you work well under these conditions?
At the end of the day you have to do what's right for you - don't feel pressured into going somewhere you won't be happy just because you are influenced by those that are seduced by Cambridge's status/reputation. Good luck!
But going to get a U!