I don't think that regions need to be of equal population, but they shouldn't be too dissimilar. California-to-Wyoming is a bit too much of a range, being entirely honest, but London to Northern Ireland is only a factor of 5 or so - that's not too bad. The trick is finding regions of England such that they have that bond. Yorkshire can pull it off, East Anglia and the West Country might, and beyond that I'm pulling a blank.
Out of pure malevolence, I've sometimes suggested devolving to English counties the same powers as the Scottish Parliament has. Probably not a good idea, but it would be fun to watch.
Tying devolution to the English regions into the original question, if Irish Home Rule is successfully implemented (either by avoiding WWI or going through as proposed in spite of WWI) then you would see Scotland soon follow (a Scottish Home Rule bill was discussed in 1913 or 1914 and was quickly forgotten about after WWI started). After this very big if, you may see devolution to the English regions or the larger counties, as mentioned by Churchill in a speech during the Home Rule crisis (I think he mentioned London, Yorkshire and the Midlands specifically).
Following on from this you may see integrationism become desired in Malta or Gibraltar (Cyprus I can't see under any circumstances) along the federalist model, but again pretty big if. An even bigger if would see the crown dependencies become fully integrated.
So the largest possible UK (not Empire) that you would see in the 20th century would be OTL UK including the whole of Ireland, plus the crown dependencies, Gibraltar, and Malta.
Anywhere else is a combination of two far away to be fully integrated (unlike France the UK has a convenient dominion on most continents to integrate colonies to a la Newfoundland into Canada) and too sparsely populated to be worth integrating (Falklands).
There may be a few more odd islands in Europe which may find themselves part of the UK (eg. the Faroes if Denmark is Fascist or Communist or occupied long-term opposed to the UK), but even the scenario we have so far presumes nearly everything going right for the UK to take territory from other European nations you take it to a whole new level.