Yeah, that's a great excuse. That should be written in the anglo-polish military alliance:
"in the event of any action which clearly threatened Polish independence, and which the Polish Government accordingly considered it vital to resist with their national forces, His Majesty's Government would feel themselves bound at once to lend the Polish Government all support in their power... Unless of course the germans use tanks and move very quickly. In that case, sorry".
The UK did not ally itself with Poland to help them from being taken over. It was a way of saying to Hitler, that he can't just do what he wants in Europe (like anecting Czechoslovakia) without the Germans and French approving it. The war was immenent, and everybody seemd to know that except for Chamberlain. This was only an act of self interest on behalf of the UK, and hey, who could blame them for that?
What you can't reasonably argue though, is that they tried to really help Poland in the beginning of WW2, and that they didn't betray them at the end in Yalta. And the occupation by the Soviets really wasn't much better than by the Nazis.
There was no real plan to take the Soviets on, because the Allies had no real stake in it.