Hi my advice is not to go for the ones like AA and BSM. The reason being (maybe others will disagree) is apparently they spend time on the side of the road talking about driving (where oil etc goes) and not actually doing any driving IYSWIM.
Where in London are you? I'm N London and I've looked in the local paper and am going to book 10 lessons for £160. I've looked up and checked them out online first.
HTH