Explanation: Just memeing! Roman Britain was actually abandoned by the Late Roman Empire in 410 AD, despite Briton pleas for them to, you know, NOT abandon them to the migrating barbarian tribes.
... the Roman Empire in 410 AD was not in good straits, and would finish its total collapse even in Italy in 476 AD.
Britain, for its part, did revert to a pretty brutally low level of subsistence with the oncoming waves of Germanic invaders. Same as in much of the rest of the former Western Roman Empire, the collapse of material technology and social complexity was near-total, with even common and necessary things like pottery reverting to pre-iron age standard of quality and quantity, and not recovering for several centuries.
The "Dark Ages" is no longer preferred as a term, because the name itself prejudices examination, and the fact that it was by no means a lack of intelligence or spirit of innovation which made the period fall so far. But make no mistake - the fall of the Roman Empire, even in its late, most degraded form, was disastrous for nearly everyone involved.