Jim,
They were given a record and prevented from working - treated as criminals, in other words

No they were not.

Being a communist in this period (as now) was perfectly legal - in fact there were a number of Communist MPs

I know, but MI5 had to monitor them because, as subversive revolutionaries, they were perceived as a threat as radical Islamists are today.

"Remember that from 1939 to 1941 British communists were in league with Hitler."
No they were not -

Yes they were.
From 1939 until 1941 the CPGB was very active in supporting strikes and in denouncing the government for its pursuit of the war describing the war as the product of imperialism on both sides, and in which the working class had no side to take.

What's the difference - they could afford to commit money ammunition and troops to support the anti-government forces.

It was a very small commitment to an international voice, before the army had changed to peacetime establishment, and in no way comparable to fighting another world war!