The Romans didnt "dissappear" in 410 actualy. What happen was about around the years of 408 and 410 some great part of Roman Military estabalished in Britania left in suport of an acailamed usuper Constantine III that went about claiming the Empire for himself this didnt meant that Rome or "Romans" left Britain. What happen in meanwhile was Romano-British themselfs were tired of their Usuper-Emperor and revolted and went to return back to proper "true" Empire but the Honorius the "true" Emperor had no real power whatsover especealy outside his little palace n Ravenna and especealy not in Britain so was when those Britons that were of course Roman get the letter from their Emperor saing basicaly just do the best you can by yourselfs because no help is coming. The Romans however never trully left they existed until anny of those Britons tough himself as a Roman and during Sub Roman Period many did so. Neither did Roman Military in Britain left many soldiers styaied and with colapse of the Empire were part of armies of Rex,Dux and other "Tyrants" Gildas speaks. Basicaly as of rest of Empire Britain slipt in petty kingdmons and factions now mixing that up by invitiation by Britons themselfs ,invasion or both here came Saxons that would become the English and name their new land England.