Yes, I forget which cavalry unit was in Ribchester just up the road, but maybe from Samaria?

However, it seems that the legions would be kept up to strength with local recruiting, and the auxiliaries could be locally raised (no-one quite so useful in keeping down the neighbours). Once settled, the legions would "embed" so uprooting them to move would bring along a motley collection of locals. Could be, of course, that things got slacker as the Empire matured?

Maximus (if it was he) intended using the British armies to set himself up as Emperor, rather than just transferring legions from one front to another. Having lost, the winner had to do something with all these soldiers/hangers-on and apparently pointed them in a north-westerley direction. It's not clear whether they were specifically put in Armorica (And Stay There!) or simply ran out of steam on the journey back. You can see why some might consider it more appealing than a return to the Wall.

So, as I see it, the legions were raised in one place and moved to another, but then settled in and became at least semi-permanent. Moving legions about did occur but was a significant event, not as regular as the the shuffling about of more modern army units.

In the records that have been found in Vindoland, aren't there comments about the poor quality and untrainability of the British recruits? Demned wogs, I say, can't do a thing with 'em.