So I just did it. I did
not replace Scotland, England, Wales, Northern Ireland as separate countries nor did I add any of the territories to the UK (which would be ideal, I guess) but besides that, I seemed to have created a United Kingdom that the game recognizes in Europe.
I'll leave it to someone else to give all Northern Irish players dual citizenship with Ireland for sports reasons, but...it seems to work. I just took the omnibus edited file from Dogzilla and added the UK as another nation. The tricky part initially was make sure it was added to Europe, but it recognizes it and I made my British League a British League instead of an English one and it works.
Take a look and see how it works for you.
https://www.mediafire.com/file/pxcmg...fault.xml/file