the reason that you don't get great indian food here in london is that there isn't a huge indian born community that lives here - indeed, most indians here are third generation now. and the original indians that came here came mainly from east africa and not from the homeland. add the bangladeshi takeover of indian restaurants and you can explain the current dismal state of
indian cooking here.
there is one place i've found however that is run by an east african descended indian with a pakistani chef that is well worth the visit - its very cheap too. that's hot stuff: it has the heavy handedness of the north with vegetables, but everything is clean and fresh. some dishes are outright winners - the paneer masala for instance.
go try it - the owner raj dawood is a real character.