Proportional Representation is a disaster. It is a prime example of chattering class insanity, an idea that has been tried repeatedly, never worked, and keeps getting dragged back for another go. If large numbers are not well represented by the current constituencies then there are other remedies.
One possibility is to create more districts. Why should there be only 650 for all of the UK? That number was created when the population was a fraction of what it is now. Are the number of seats really to be determined by the number of places to sit within Westminister? The natural size of a democratic district should probably not exceed that of the Aristotelian ideal polis, about 50,0000 voters.
Another possibility would be to spin off Scotland, Wales and Ulster and free up those seats for distribution within England. If the UK is just a province within the EU then those regions do not need an intermediary legislature between them and Brussels. Personally I would rather see the UK free of the EU, which is a sinking ship anyway, and most social policy controlled by local assemblies, perhaps 4 or 5 within England, with only foreign affairs and trade controlled by the Union Parliament.