At a Foreign Policy Committee meeting of the British Cabinet, the Ministers decide to side with Poland, rather than try for a multi-nation agreement involving the Soviet Union.
http://www.worldwar-2.net/prelude-to-war/prelude-to-war-index.htm
Update at #12.
There are a number of reasons for this:
We note: that at this point Buchanan says, the Brits should have urged Poland to do like Czechoslovakia. His basic argument is that when all was said and done, Poland lost nearly six million dead fighting the Nazis, while Czechoslovakia lost 95% fewer by cooperating.
In 1939 Poland was three times larger than Czechoslovakia. Today, even with new borders it's four times larger than the Czech Republic.
I think the Poles' fate was sealed, especially the Jews, regardless of how much they might have tried to cooperate with Hitler.