According to ztl there are two holders in England, Axe Valley and Wingz so not easily accessible for you @FunkyGibbon. There are an added 8 holders throughout Europe as well so there's potential for them to spread a bit more.
This kind of thing really irritates me... why do zoos label animals in their collection as something they are not?
That said, I was kind of wondering already why the "Japanese squirrels" I saw in real life or in photo from these places all have these white patches. Them being some kind of color morph would explain that…
I saw piebald European Red Squirrels in private collections in NL in the 1980s. Then, they were called just that. 'Japanese' Squirrel appears top be a new name...?