How did the British think operation Barbarossa was going to affect their war against Germany?

As the German invasion of Russia was about to happen, and as the invasion unfolded, how did the Brits perceive this? We’re they glad to have another ally with whom they could defeat the Germans, or were they fearful that Germany would win and become too big to stop? And since they were ideologically opposed to the USSR as well, did they the at the same time perhaps worry what would happen if the Russians won, and what would happen if the Russians would end up controlling central Europe?