If the majority of the population of Northern Ireland voted for independence and the majority of Ireland as a whole voted for reunification, the UK (you mistakenly keep referring to England) would not stop it. There's a reason why the IRA now spend most of their time knee capping teenage drug dealers than they do blowing up shopping streets and that's because most people in Ireland (including former IRA members) prefer politics and democracy to random acts of violence.Look at the support to kick out brutes/brits from Occupied Northern Ireland.
As for Scotland, it wasn't invaded, it joined with England after the Scottish king becoming the King of England and later through the Act of Union, through which the Scottish parliament voted to join England and create the UK.
They also had a referendum in independence fairly recently (from the UK not England) and chose against Independence.
The Welsh equivalent of the SNP, Plaid Cymru don't poll particularly well in elections in Wales and there isn't a particularly significant appetite for independence from the UK either.
Ironically there was an amusing poll before the Scottish independence referendum that showed more people in England in favor of it that people in Scotland.
Oddly England was also a Celtic country, the natives didn't vanish or get replaced, although the culture was more significantly overwritten by Anglo-Saxon invaders than anywhere else in Britain and Ireland.
Continue to fight the good fight of the 1980s! Though the rest of us have moved on.