I believe the answer to this without going into too much detail, is that when Britain and France declared war, they were not really ready for a war in Europe. They had only just started re-arming their country, war was unpopular among the people, and they knew their economies would not be able to sustain a long industrialised war. I also don't believe the British knew how to react. Russia was a traditional ally of Britain and a tradition enemy of Germany. When Britain appeased Hitler in the Munich agreement, they were certain that peace had been attained, so it was a shocking about face for Britain when they realised that there would be war regardless. By declaring war on Germany, they declared war on it's allies too.

It's also notable that there was considerable discussion given to sending a British and French expeditionary force to aid Finland in the Winter War, which would have been a de facto declaration of war with the USSR.

(Fun fact: When British troops went to Korea in 1950, the winter boots they took with them were the ones that had been stockpiled ready for the Finnish expedition.)