I went to college in Washington, DC, which, was similarly bustling. I don't have sensory issues, but knew people who did. They wore headphones or earplugs as a socially-acceptable way of tuning others out. You can listen to actual music, but you can also listen to nothing. If you don't own them already, I'd consider buying noise-cancelling headphones, as they seem like they'd benefit you.
I have been to London, and really enjoyed walking around at night. Unlike other cities I've visited, London has a bedtime, so the city is much quieter when the Underground shuts down around 1230. Of course, it's very safe there.
Oh, and for those obnoxious tube rides, here's an idea:
http://www.nbcwashington.com/news/local ... 96286.html
Best of luck! UCL is a great school, and London is truly the greatest city on earth.