The British counterpart to HUAC was the assumption that anyone in a trade union was a communist and anyone who who went to public school couldn't be. So the security services spent a lot of time on the labour party and missed the Cambridge Four because they were "the right sort of chap".
Another anecdote: In the 1950s the KGB thoroughly infiltrated the British government and security services, but their assets would send back would be public school English so convoluted it literally could not be read by their handlers at times.