The Ace
Scottish Roman.
I know little about this particular conflict, but there was no English Army until Cromwell's New Model Army. You had mercenaries, who were fighting for cash rather than for King and country. Their allegiance was only to the particular Lord that was paying them, and it stands to reason that they would be recruited locally, or as locally as possible. We forget how vast the distances were in those days because today we can make a weeks journey in a matter of hours.
The King'd raise an army at need by calling on his vassals. Medieval armies had little in common with modern ones, but they were made up of men summoned by the king. There were mercenary companies, of course, but kings found patriotism cheaper.
There were Scots in the English army at Stirling Bridge (including the future Robert I) who either owned land in England, saw an eye for the main chance, or just didn't support Wallace.
Medieval armies dispersed at the end of the campaigning season or when the grub ran out, rather than being a permanent feature like modern ones, but they were organised by the nobility calling their own men to support them, often at the behest of a higher noble or the sovereign.