One would only choose York, Leeds or Sheffield if one chose to ignore regionality completely, and go instead for increased local devolution, to County level.
The reason for suggesting Newcastle is its historic geography in relation to the Debateable Lands, which we well know here. The Midlands are already over-represented in the model I gave, and adding Leeds or Sheffield, or even York, leaves a howling gap in the North. This was a past mistake not to be repeated.
But as it is, you miss my point: regional devolution was voted down a while back, and will not be back on the table for a very long time, certainly not in the next half century. The model that is working is Scotland, is Wales, is Northern Ireland: which only leaves England, at an intermediate level.
Yes, you could argue that the other groups are a fistful of millions, and england is very diverse. The South East on such a basis would be the equivalent of several Scotlands, and unless we were to take London separately from Home Counties as a ring around it, then any other separation would make little sense. Even then, living within easy walking distance of the northern edge of Greater London, the fact one can cross a road and find oneself outside the boundary without seeing any social distinction, makes it an illogical distinction.
It almost makes more sense to revert to the Anglo-Saxon kingdoms grosso-modo, Wessex, East Anglia, Mercia, The North, Home Counties.