I'm not sure what the laws regarding same-sex marriage or visa sponsorship for non-married same-sex partners are in Britain, or if such things are even relevant to your situation. I just want to make mention of the fact that neither are possible in Japan at the moment and in the event of your partner deciding to return to Japan there is the possibility of things becoming horribly complicated for you.
Not saying yea or nay to your intended course of action, just want to make sure you have given thought to all possible repercussions from it. The law of unintended consequences has a way of rearing its ugly head from time to time, you know.