The last point at which England could realistically balkanise would be during the War of the Roses.
As for a United Ireland that's difficult. The truth is there has only ever been a united Ireland for any period of time during the time it was controlled by England. If you were wanting a unified nation of Ireland completely free from English control then you're looking at a POD in the middle ages
The South West of England is Celtic, specifically Briton as is Wales and Brittany. Scotland, Ireland and Man are Gaelic and btw, the Gaels and the Britons got on about as well as the Anglo-Saxons and Britons, or for that matter the Gaels and Anglo-Saxons.
As for a United Ireland that's difficult. The truth is there has only ever been a united Ireland for any period of time during the time it was controlled by England. If you were wanting a unified nation of Ireland completely free from English control then you're looking at a POD in the middle ages
The South West of England is Celtic, specifically Briton as is Wales and Brittany. Scotland, Ireland and Man are Gaelic and btw, the Gaels and the Britons got on about as well as the Anglo-Saxons and Britons, or for that matter the Gaels and Anglo-Saxons.