UK (and Canada, Australia, etc.) did declare war on Finland, and attacked a Finnish harbour[1]. While USA didn't declare war, I doubt anything would have changed if they did, since Finland was out of their reach anyway.

[1]: http://en.wikipedia.org/wiki/Raid_on_Kirkenes_and_Petsamo

http://en.wikipedia.org/wiki/Continuation_War#Finland_and_th...