Under normal circumstances, the ruling party always chooses the Prime Minister under a parliamentary government. Thanks for reminding me of this point.
Since this is a what if thread, I thought of considering Atlee since the politics at that time was in flux. If we consider all possibilities, I think the King could've ordered the Parliament dissolved and called for new elections to settle things. Correct me if I am wrong but that's how I understand how a parliamentary government operates. I understand that England operates under a constitutional monarchy. In this context, is my opinion plausible? Your reply would further my understanding of the UK.