There are three Lolita shops in Camden: two Gothic Lolita & Punk (or GLP) - one is one the main street just... after the bridge? I think. And the other is in the stables market - and SAI SAI, which is also in the stables.
The products are pretty hit and miss though, as they stock brands like Punk Rave and others that you can find on ebay. Mostly it's the lace that makes the clothes look somewhat bad, but you can always remove that later I guess. They do stock an*tai*na shoes though, so it's definately the place to go for those!
Have fun shopping!
EDIT: I reread your post and realised you meant other than in Camden. There aren't any other 'official' Lolita stores in London. You'll just have to search through the highstreet shops and charity shops for hidden treasures and loliable items. There's always something (: