Well I live in Bromley when I'm at home, which is roughly Zone 3ish, and it takes about 20/25 minutes on a train to get to Charing Cross, which is where the main KCL buildings are.
I'm also at Bristol uni, and absolutely love it - cannot even begin to express how much fun the city is! There's always lots and lots to do, and it's so so so much cheaper than London. EG in London I'll usually be charged £5.50 for a single spirit and mixer or small glass of wine, but here in Bristol I'll usually pay under £2.50.
Housing is another monetary issue - in London you'll be paying easily over £100 for a single room per week that isn't terribly nice, whereas I'll be paying about £85 per week to live with friends in a beautiful 4 bedroom townhouse with kitchen, dining room, conservatory, utility room, garden, several bathrooms etc.
In all honesty, I prefer Bristol to London as it's a bit more manageable and student friendly, not to mention cheaper! It's still an 'expensive' city to live in (in comparison to somewhere further out or up North) but you do definintely get more for your money.
Hope this helps