Firstly, thanks for this thread. This solves an issue I was having a few years ago, going to have a try at remaking that db now I've seen this. I just wanted to clarify something about your post above while I'm still planning the league pyramid and haven't started making the db.
I was thinking of having a joint league system and then below that the current pyramids of England (starting at National League or National League N/S - ideally N/S so I have 4 leagues at that level but can do NL if I need to have 1 league per region), Scotland and Wales (both starting at top league). Each nation's pyramid obviously splits off in different ways, has a different number of levels and different numbers of leagues at each level. Everything below the level of the split would have normal promotion/relegation as happens currently.
Would this mean every country would need to have the same number of levels below my split, the same number of leagues only at the level below the split or the same number of leagues at each level below?