Thank you. The whole point of the deal from Churchill's perspective was to give up those areas Britain didn't have a chance of influencing in exchange for guarantees that Stalin wouldn't mess around in Greece. He succeeded. He gave up something that he couldn't have had in order to get something that he needed.It was a good deal; 50/50 influence in Yugoslavia, where Britain had very limited capacity to do anything, it lost Rumania and Bulgaria (which were inevitably going to be dominated by Russia) and it won them Greece, where the Russians could easily have made things far more difficult for all concerned.
With regards to Churchill's relations with Stalin, nothing that I've seen suggests he had any illusions about the mustachioed Georgian.