The main problem was, that Poland and USSR were not in the state of war.
The Polish government believed that Soviets will stop the aggression and forbid Polish troops to fight against Russians. It was because of a non-aggression pact since the peace treaty of Riga in 1921. The Poland did not want to break this treaty. Every assistance requested by Polish government was to make Russians withdraw from Poland, not to make war against them. The British diplomacy failed (or did not take any actions, I can't remember), but it was not the only "success" of British diplomacy since militarization of the Rhineland.
Later then, because the diplomatic relations between Polish 2nd Republic and USSR were not broken, the Sikorski-Mayski agreement could be made, and POWs from Russian part of Poland could go through Iran and Afghanistan to British India and Palestine to Egypt to fight Germans.
The diplomatic relations were broken after the revealing of Katyń Massacre; allowing thus to Stalin make a puppet government in People Republic of Poland. For British government, the Polish one on exile was then not necessary because USSR was now one of the main Allies, much, much stronger than occupied Poland.
And personal note...
As I remember from (communistic) school, the Soviet action was named "intervention to protect working class and peasantry against German invasion". There we no photos of Soviet and German troops fraternising on the Polish territory, as "they were enemies". Officially, the USSR saved Polish people and was Germany's enemy since the very beginning (1939).