That’s very difficult to answer with any degree of accuracy – no records exist from this period that allow anything but a relative figure to be gained. Needless to say England was massively more populous than the others, though the population was still a good way behind its pre-plague high of c.6 million. We can only perceive trends in the population at this time though an education guess might say 3.5-5 million in England. Scotland & Wales both have relatively small areas of decent plough land, much of the geography being steep, stony or boggy, hence could not sustain much in the way of population. It is unlikely that Wales had a population that even approached a million – its revenue yield was less than many English counties. Scotland, though more wealthy & populous than Wales, still probably had a population less than a quarter that of England.