I read somewhere in the grand, often-depressingly-inaccurate wilderness of Wikipedia that many British individuals in particular attempted to downplay the Soviet invasion of Poland as being morally just.
I do know, however, that eastern Poland was largely inhabited by Belarusians and Ukrainians who probably would have felt more at home within the Soviet government, and that the invasion may have been somewhat of a liberation for these groups.
So - were British citizens just trying to protect relations with the Soviet Union - a major trading partner which was important to fuel the war with Germany - or was it a logical viewpoint?
Regarding the Belorussians, any that would have felt more at home in the USSR, would have moved there already. There was a propaganda campaign started under Lenin after the end of the war with Poland, in order to strengthen (if not outright create for the first time) Belorussian nationalism. Soviet policy in the 20s was to show that the BSSR was a better place to live in for Belorussians than in the polish eastern territories, both in order to pacify the region and to undermine the Polish government. The May Coup led by Pilsudski in 1926 which resulted in him ruling Poland as a dictator only reinforced this narrative, and large numbers of people left the eastern territories to the BSSR. Stalin however would reverse these policies as he grew to see Belorussian nationalism as suspect and a possible cause for treachery.
If you're interested, Per Rudling gave a great overview of the period in an interview on his book The Rise and Fall of Belarusian Nationalism, 1906-1931.