Britain & France weren't interested in Poland for territorial gain, the interest was purely diplomatic. They wanted hostilities to end in Europe before war became an inevitability. At this time WW1 was still (relatively) fresh in the minds of the leaders as well as the public, they would (and did) go to some lengths to prevent a war. Later it became clear that a war couldn't be avoided without basically handing Europe to Hitler.

Britain, France, and Poland signed a cooperation pact before WW2 broke out stating that if Germany attacked any of those 3 countries, the others would declare war and intervene. Unfortunately, whether it took a while for Britain and France to get mobilized, or whether "Western Betrayal" was the actual reason (B+F didn't want to honor the pact/more appeasement) , there was a delay in military response from those two countries when Poland was attacked.