Having the Romans take over the whole of Britannia by 450 would be difficult, even unifying the old Roman parts of the island would probably require a very capable general who becomes one among many claimants to the Roman throne, at least initially.
That being said, if it can be done, the Romans would be in a good position to eventually consolidate and invade first Hibernia, then Caledonia, and eventually turn their attention elsewhere. I could see the Romans eyeing the modern Low Countries since they could be easily defensible and have richer farmland, though Brittany and OTL Normandy would appeal simply for the sake of proximity. Landings near modern Bordeaux in Aquitaine would permit the use of a river to the north and east along with mountains to the south as initial defenses, and if the Iberian peninsula is thought to be weakly held then maybe they decide to land there instead.
Interestingly this could lead to Roman fisherman discovering Iceland then Greenland, and if there are sufficient refugees from the mainland you could have a post-Roman kingdom with enough people and technology to have a massive advantage over its neighbors, especially following the Justinian plague. Contact with the New World would be plausible if they get that far, especially with rich fishing grounds at the Grand Banks. Such a kingdom could develop a strong oceanic navy and become a check, if not a threat, to the Vikings in time. It would also probably claim the whole of the western Roman Empire (if not the whole thing) in name, though it might only seriously try to hold Gaul and/or Iberia.