The rule isn’t that ICs can’t exist on islands. It’s that they can’t be “built” there (pg.23, pg.27).
Ohandbytheway, according to pg.8, “an island or island group is a single territory surrounded entirely (emphasis mine) by one or more sea zones” - and surrounded by seazones means not bordering other territories. Therefore, the UK is not an island. For that, matter, neither is Scotland. The Brits can build a minor on Scotland. For that matter, so could the Germans.
Japan is an island. And that means nothing, considering that scramble rules are no longer limited to islands. You can’t build an IC on Japan, per the rules. Big deal - it already has one. The distinction of it being an island means absolutely nothing anymore…