There's an old story about sombeody like Manny Shinwell (a CP member in the house of lords) and somebody in the labour party convinced they were being bugged and speaking Yiddish to each other on the phone (as if MI5 couldn't find somebody to understand Yiddish, polari, you-name-it)

But being a communist does mean you are more likely to be recruited if you are in a position of power and at a time of 'war'.

Wouldn't that be an an too obvious foreign agent?

Just pay a 100% capitalist

Debatable since foreign powers were clearly trying to grab influence thru the funding of communist parties in western countries