British spies are some of the best in the world, just because China claims they have gotten rid of all spies doesn't mean they have. I mean the US has caught several Chinese spies but lets not kid ourselves that there aren't still Chinese spies in the US.
They used to be the best, at least until the 1970s. But I really doubt whether that is the case right now. Right now, the British supremacy in the espionage exists only in the 007 movies.