What if the 20th of July 1944 Hitler has been "terminated"

[TinCow]

No, but we did declare war on Germany to protect Poland. Rightfully or wrongfully, or forced by circumstance, in the end, we picked one of the two 1939 invaders of Poland for an ally, and sold out Poland to him.

The 'what if' here, is there a possibility that we would've sided with Germany against those who occupied Poland? It is, after all, what we did do after, say, 1949. What was the earliest date possible? Could it have been done in 1944, at the possible prize of giving Poland to Germany instead of to Russia?

No, the UK had no interest in fighting the Soviets over Poland at any point that is relevant to this discussion. The UK-Poland military alliance was specifically targetted at Germany, not the USSR. In reality, the UK didn't care a whole lot about Poland's independence, they just cared about limiting Germany's expansion. The exact same was true of the Franco-Polish treaty. If you want to see how the UK/France would have responded to a unilateral Soviet attack on Poland, it's probably more accurate to look at their response to the Soviet invasion of Finland. In that situation, the UK/France made a big diplomatic scene, but didn't have any intention of opposing the Soviets militarily. The Cold War situation is not remotely comparable to WW2, and is more in-line with the UK/France reaction to the Finnish invasion. They were certainly not willing to risk war with the USSR in 1949 (or at any other subsequent date) over Polish independence.