I have just read this answer and would like to know if there is a known reason why the British did not inform the Poles. Or is the whole thing wrong?
You have to take into account the source. The quote that makes this claim was from the memoirs of a USA diplomat to Russia. While this may have been his view of things, he can't possibly have been privy to every secret communication that went back and forth between English and Polish intelligence. He was also spending his time in an environment (Russia) where a lot of folks had a vested interest in pushing stories that made Russia's part in things look better than they were.
Clearly Poland was worried about attack from Germany, or they wouldn't have signed the Anglo-Polish military alliance in 1939 the first place.
According to Churchill's memoirs of the war, the Polish government refused to make defensive arrangements with Soviet Russia in the months before these events, because they were afraid the Red Army wouldn't leave once invited in. This refusal to cooperate (again according to Churchill) was precisely what drove the USSR to cut their losses and sign the pact with Germany. Given the previous attitude from Poland (refusing Russian help even to save their own skins), I doubt much of anyone in Poland had any doubt whatsoever that the public announcement of the Molotov-Ribbentrop pact along with the upswing in anti-Polish rhetoric from Germany, meant an agreed carve-up of their country. As they say, you don't have to be a weatherman to see which way the wind blows.
What's more, in the actual invasion, the Russians didn't coordinate attacks with the Germans. They just marched their troops into the alloted areas while the Germans beat the Polish army. If the agreement had been for the USSR just stand aside and do nothing, getting no territory in return, the result for Poland would have been exactly the same.
So personally I find the claim that the Poles were somehow maliciously not informed about the proposed carve-up, and thus things turned out worse for them, unlikely on both counts.