The British Chiefs of Staff were not surprised at the German attack on Russia on 22 June 1941. The British had been weakened and were not a threat, but were strong enough at home that an invasion was impractical. The British were aware that there was a large scale movement of German forces to the East. The reduction in German air activity in the Mediterranean theater and over Britain was also indicative of the shift in forces elsewhere.
Rumours accurately forecast that the Rumanians would attack Russia with the Germans. By late May, the German armies facing East had grown to at least 100 divisions. The British expected the Germans to go through some exercise where they would demand concessions from the Soviets that they could not accept, and then use that as a pretext to invade. The reality was the Germans skipped what used to be the accepted forms and just attacked without warning on 22 June. This is based on the account in Vol.II of the Official History.