You will have to change all references to the ENG tag (not just those in major_eng.txt) that occur after the tag change. And if you don't make the tag change a sure thing (as it is in AoI), eg. by requiring that England owns or is buddies with Scotland, you will have to duplicate any events that happen to ENG after the tag change. You can, I think, get away with just adding duplicate references to your chosen UK tag in events that fire for other countries, but I suggest adding "no = (UK tag)" to ENG's entry in revolt.txt, so England can't revolt if the UK exists. That should prevent weird stuff happening because both exist simultaneously. Of course, if any events trigger off of ENG events (or the existence of ENG, or whatever) when ENG might actually be the UK you will have to change that to an OR trigger, referencing both the ENG and UK versions of the event.
Phew. Still think it's simple?