Perhaps a few of the following might help in your overall understanding of the situation in the late nineteen thirties.
France and Germany had promised Poland that if she was invaded by Germany, as had Czechoslovakia, then F and GB would come to her aid and defend. Germany invaded and thus the promise was instigated.
Russia was not a 'natural' enemy of Great Britain in the sense that Germany had been biting our heels for years, not least the Great War itself. So it could well have been seen that Russia was holding back the inevitable German invasion by becoming friends with Hitler to give Stalin time and space to re arm and get his act together. GB by virtue of the fact that war was agin Germany and all her allies, ie the axis, meant we were not in the same bed as Stalin at that moment.
Later when Germany decided to rub out Russia Stalin switched sides and then began to use the allied forces to defend the Russian Motherland, and many historians believe that was always his primary intention closely followed by an expansionist policy in line with the Communist doctrine.
Stalin was not on Churchills Christmas card list as his worry was that maybe together we would win the war but would there be Russian troops at the Pas de Calais in so doing. A tricky balancing act for Churchilll to manage.
Therefore to relate to your original point there was no formal declaration because the war politics were so fluid. Hitler never intended to be friends of Stalin, the Allies preferred Stalin in the team than outside it but it all had to be skillfully managed.
All in all I think we did well even though Russian territory was expanded considerably at wars end. Now of course it has changed yet again.