What happened to Nero from Yorkshire

I was taught that the Romans left Britain in 410 AD to go to defend Rome. Well, it seems the army was about 500,000 strong and maybe 100,000 were in Britain, so where did they all go? 500,000 soldiers crowding into Rome all at once? They must have been standing on each others heads.The ones that had been in Britain and elsewhere for four hundred years would have women and children, so we are talking more than a million refugees all cramming themselves into one walled city in Italy. No wonder the Italians are unhappy about the present influx. Did the Brits who had joined up go away, or stay behind. Was it like the aftermath of WW1 with all the young men gone and more women than men in Britain and more men than women in Italy? There really are a million stories in this, not stories about wars and heroes but tales of families left behind. Were all those soldiers killed, or did they go home and become outlaws. The Roman empire moved to Constantinople so, are the present immigrants the descendents of Roman soldiers trying to go home? Maybe one or two could claim British passports because their ancestor was the Yorkshire servant of a centurion in 45AD. This new fad of DNA history could backfire in ways we didn’t expect.