Although much of what he said should be treated with skepticism, a short 2000 Guardian piece written by the KGB defector Oleg Gordievsky asserts that the British school system produced spies with the necessary interpersonal skills to become a good agent. He ranked the British as the most competent, the CIA as middling due to the competition from the private sector, and French intelligence as incompetent. http://www.theguardian.com/uk/2000/apr/28/military.davidshayler

Keep in mind though that Gordievsky does need to be treated with skepticism. He made a number of unsubstantiated claims about Soviet infiltration of the West that resulted in a libel suit settled out of court. And since MI6 handled his defection, it is in his interests to play up their efficacy. And the admiration for the British was not universal among Soviet intelligence. One of the KGB's historians Mikhail Lyubimov is more dismissive of British efforts, especially after the Philby affair.