the deal was, Poland was supposed to hold out for a month to get the Brits some time to get over to them. the Poles couldn’t last that long
Kind of hard to do when you're invaded by two countries.
Poland held out for at least 5 weeks; Britain was unwilling to go to war with both Germany AND the USSR (since both invaded Poland). Britain proceeded with the war, losing countless lives, only to see Poland emerge as a Soviet satellite at the end. Churchill in particular knew the Brits had lost so much blood and treasure for nothing at all except to lose their empire as well.