Coming to this thread a little late, but will chip in anyway...

While Sree Krishna is good, I prefer Radha Krishna Bhavan for South Indian, which is also closer to Tooting Broadway tube. Personal taste of course, but excellent dosas.

For a more centrally located S Indian, try Ragam at 57 Cleveland St in Fitzrovia. It's very small though, so can fill up quickly.

Recently had a good meal at Namaaste Kitchen in Camden. Service is reported to be a little variable (was fine when I was there), but food is very good.

Although the small local places are usually the best, I do also think that the Masala Zone chain does pretty decent food.