There were different 'political considerations' involved based on events in the OP's game which unfolded differently than in real life. I don't know what 'rank' Russia would have been considered at the onset of the Russo-Japanese War, but given their clout at the time probably higher than #7. And most of Europe didn't yet consider the Japanese Empire a credible threat to the Great Powers. So the British choosing to side with Japan in that scenario is an example of seizing an opportunity to weaken the larger threat. Pretty much exactly what they did in the game, except in reverse.
And yet IRL they didn't switch to supporting Russia after Japan proved itself to be the greater threat. Great Power politics wasn't all about allying with countries which could help fight against the nations threatening you, it was often about allying with those very same threats. As a friend, they stop being threatening. Hence things like this.