You have given technical explanations, but the practical explanation is that you just can't be bothered to accommodate your users. I doubt that it would be impossible, even without an ISO number, for Scotland's flag to be uploaded from the net and included with the others. Technically, there is no such country as the United Kingdom. It is a collection of individual countries. If you can find all the other flags, don't stand behind technicalities on this issue. Just do it.
It's not too much to ask since there are a number of Scottish people playing your game,