I would still be much more worried about the GRU. The head of the MI5 from 1956 to 1965 was likely a GRU mole and he got away with it. Who knows what other moles they had. They did not open their archives in the 90’s like the KGB did.
Might a story like that have been the inspiration for Tinker, Tailor, Soldier, Spy?
” They did not open their archives in the 90’s like the KGB did.”
What the KGB did open was interesting enough....