If you are willing to squat by yourself i would advise you, go to trafalgar square. Take bus number 29 to Woodgreen, than change to 121, 141, or 329 to Enfield, leave on North Circular Road(or something like this) station there will be a highway called by the same name. There are almost 400 houses along the highway that you can squat, and there is more than 30 squats that i know in that area. (there is even few documentarys about those houses).
There is some squatable blocks near Finsbury park, but they are run down crack places...
Havent been in london since august, so its just what i know... things may have changed. Good luck
If you'll have no luck in london, try Bristol. It has a pretty decent squatting scene and people are a bit nicer because living conditions are far more better and there is lesser competition at dumpsters etc...
How long are you willing to stay there?... i think im going to Manchester on February/March with friends so if you'll still be in britain at that time and the situation will be shit, message me. Maybe i'll have a place for you to stay there...