Wasn't a requirement for either participant of the Anglo-Japanese alliance that they had to be at war with atleast two major powers before they could join in?
gigalocus
In the 1st version, which lasted until 1911 yes that was the case. However that is if the ally is at war with two powers they are committed to joining in to help them. Nothing to stop Britain joining a war on Japan's side if it wants to.
Since Japan is an ally and also Britain is becoming worried about Wilhelm's attitude and naval programme I could easily see at least a call to help our 'gallant allies'. This could include technical help, supplies, funds, etc. Or even some quiet words and naval manouvers in the North Sea that could have interesting effects on German ability to deploy forces, gain loans etc.
Mind you it may not be needed. The front is a lot closer to Japan, which is already recognised as the legal owner of the territory in question. As such, like with the Russian Baltic fleet its going to be a long haul with great fun getting supplies. [I doubt their going to be able to use French colonial ports like the Russians were

].
Furthermore the date would be important. If before the OTL Russo-Japanese conflict, which could then be butterflied, then the Germans will probably vastly underestimate the Japanese. If after it they will be more cautious and possibly have a larger fleet available but not only will the Japanese also have more ships built for them but they also renovated many of the captured Russian ones.
As such, while Britain probably would step in to some degree or another it may not be needed.
Steve