They wouldn’t have needed a spy, they just would have needed to read a German or Soviet newspaper, as the pact was announced publicly. The pact did contain secret protocals for dividing up Eastern Europe between them which did not become public until after the war, but they definitely knew about the non-agression pact, which was enough.
England and France probably did know, in advance, about the 1939 Nazi-Soviet Pact but I can’t find anything to prove they did.
Everyone would have been spying on everyone else in 1939.
There was little or no trust between nations.
The website below gives a good account of what we are told happened.
They wouldn’t have needed a spy, they just would have needed to read a German or Soviet newspaper, as the pact was announced publicly. The pact did contain secret protocals for dividing up Eastern Europe between them which did not become public until after the war, but they definitely knew about the non-agression pact, which was enough.