My history is a little off, but didn’t Great Britain declare war on the Finns.......?
It is a little tough to follow just whose side the Finns were on during WW2.......
Their own, of course. They learned in The Winter War that if you need a helping hand, the best place to look for it is at the end of your own arm. They also knew who their real enemy was and were co belligerents with the Nazis against the Soviet Union. GB declared war against them for that reason but surprisingly that commie FDR did not.
The Finns were in a pretty percarious situation throughout the war. Right now the Allies are sympathetic to the Finns since they are being attacked by the larger Soviets. The Soviets are not direct enemies but are certainly not allies either. This is a point where the entire makeup of the war could have shifted immeasurably. Imagine if the British and French at this point had decided the the Soviets had full thrown in their lot with their current German allies. The Finland issue may have brought them to declare war on the Soviets as well.
By mid 1940 the Germans began supplying the Finns with weapons though it was covert at first. The reason for this was they knew that the Finns would readily jump in on a war against the Soviets that had Germany as a major participant. Romania was in the same boat at this point. Both countries wanted the land lost to the Soviets in 1939 and 1940 back and were willing to align with the Nazis to get it. In fact, when there was a coup attempt on Romanian leader Antonescu by the Iron Guard in January of 1941 Hitler stood with Antonescu to maintain the Romanian alliance and only tried to keep too many of the Iron Guard members from being shot by the Romanians.
Needless to say when Finland assisted Germany in the invasion of the Soviets it was only natural that the British would declare war on them.