I know of at least 4 kosher Indian restaurants in London, including Beit Hamadras, Kavanna, Matancherry and Moses Tandoor. I havent eaten in any of them personally, but have heard good things about all of them, especially Beit Hamadras.
They are all meat- I do not know of a dairy one, and I have attached a link to the restaurants in England for you.
I stand corrected. All of these must have opened since my last trip to London, in 2004. I look forward to sampling some of them next time I'm there.
As for Indian dairy, we have plenty of that in NYC; these are normal non-Jewish-owned Indian vegetarian restaurants that have got a hechsher. That way they don't depend on the kosher crowd for their livelihood, and they don't have to close on Shabbat. I don't know why nobody in London has done that. It's much easier than doing a meat place.
I'm not aware of *any* kosher Indian restaurant in London - good, bad, or indifferent. Which is surprising, considering how deeply Indian food has penetrated in the UK - I believe chicken tikka masala has displaced fish and chips as the iconic English takeaway food. Surely with an Indian food market that size, *someone* must have thought of going kosher.