There is surely a lesson to be learnt from history. The UK offered certain assurances to Poland before WWII then backed off when it realised that the heat was too much.
This has been debunked. The French and English did not intervene in force because Poland fell unexpectedly fast. If Ukraine had fallen in a matter of days, there also would not have been Western help.
Your focus is wrong. Poland made a number of political decisions based on British advice and promises. That the British and French couldn't lend a hand in time, is beside the point.
In the UK's defense, 2 days after Germany's invasion of Poland, the UK declared war on Germany and started a blockade against Germany (which persisted till 1945). The next day, the UK bombed German warships in German harbors.