London is probably my favorite city outside of the US (and I've been to a few) but the price tag along with the currency exchange puts a lot of backpackers
off.
The great news is that once you get there, you can get a cheap flight to explore just about anywhere - its safe to say that London is the hub for that side of
the world.

Patrick Seery, the editor of
http://www.inlondonguide.co.uk/ wrote a good article full of tips for places to go without blowing your budget, here's
his new article on the site:
http://www.startbackpacking.com/travel/tips-travel-to-london-cheap/Cheers!