Camden High Street, from Camden towards Mornington Crescent you have
Oxfam - quite good for designer clothes but you have to trawl through the average stuff to find it, occasionally a real bargain
Cancer Research - some nice things but the designer stuff is top dollar for charity shops
British Heart Foundation - Very high prices sometimes more expensive than new
Scope - a wide selection of clothes with some bargains and some over-priced. They have eccentric taste in clothes, so some things that are just awful are expensive and some nice stuff is cheap. Higher prices for obvious vintage / retro clothes than Camden Market
Help the Aged - mostly average clothes with an occasional bargain as they don't have a clue about designer stuff
Age Concern - They have absolutely no idea about pricing, much is overpriced but occasionally a real bargain. Will only take cash
eta
It's worth checking out the Stables part of Camden Market
If you're friendly and praise their stock but plead poverty you can almost always negotiate a massive discount
I bought a load of stuff from one stall for 30% of the stated price, that was a bit of a one off though, I often get 50% discount but I go there a lot