I live in Switzerland but I go to the Kovalam every time i'm in London. We've visited Kerala a number of times, and spent a lot of time in India, and this is not only the most authentic restaurant we've been to outside Kerala, it is in fact better than most IN Kerala! The salted lassi is the best I have ever tasted, anywhere in the world! and the masala dosa is a real delight. Beetroot thoran was a real discovery - amazing! Only disappointment was the friend crab which was way too spicy, but the friendly staff changed it for me, so that was a really nice surprise.
Highly recommend it - would give it 6 out of 5 if I could