Also, look how narrow the margins are on that scale bar. Most of those regions would need to be further subdivided, and the whole of the UK would end up like Baarle-Nassau: Baarle-Nassau
https://maps.app.goo.gl/aRn73rm1cz2fHznt7?g_st=ic
The British Empire certainly did have a lot of experience of that, but it wasn't exactly good for the people on the ground where that had been done. One of those partition lines led to a 30-year low-intensity civil war that only ended in 1998: https://en.wikipedia.org/wiki/The_Troubles
The UK has plenty of experience drawing partition lines in other countries. I have full faith that they could manage it here if they tried.
Looking at the map, these would even be very clean lines by British standards.