[utsu]

IB is basically a glorified high school diploma. It means very little, really, and is mostly useful if you want to study in an English-speaking country.
It will not "recognize you as a native Anglophone of the UK. " That's just pure nonsense.

As it's basically a high school diploma, you will not be eligible for an employer-sponsored visa. For that you need a BA.

And seriously, getting married at what? 20? just to gain residency in Japan is just plain foolish.

And even after that, how do you plan to make money once you're married and living in Japan? Or is that left for your future husband to worry about?