The two main reasons that Great Britain was opposed to the Soviet Union's occupation of Poland after World War II was...
If Poland was taken by the Soviet Union, trade would be harder to accomplish.
If Poland was taken by the Soviet Union, that would mean they would be under communist control.
Poland was essentially a political country during World War II, whoever had control of Poland was seen to be winning politically. Great Britain, along with other countries like the United States, wanted to prevent and stop the spread of communism at all costs. They wanted to protect Poland and make sure it did not fall under communist control.
When Germany invaded Poland, Great Britain was quick to go to war with Germany and try to stop Poland from falling to another country. Germany had signed a nonaggression pact with the Soviet Union, which only made matters worse.
Britain was against communism, and the Soviet Union in Poland made the country a communist one. As well, they were losing trade with the country due to Soviet intervention in trades.
