[Menydh]

After the fall of Rome, the regions which had been under an intense process of romanisation retained much of it. Roman-style administration remained to varying degrees.

I'm not sure for other parts of the Roman Empire, but in Hispania the Hispano-Roman senatorial aristocracy tried to keep it as much alive as possible. The new Gothic nobility already had some degree of romanisation when they arrived, even if small, and they inherited the structures of Roman administration.

This is in the grand picture. However, some authors assert that in the areas which had been subject to a less intense (or nearly none) Roman influence, there was some degree of return to the pre-Roman gentilitium (tribal) societies. Also, the Goths introduced some of their own tribal customs, and were not fully able to keep up to the standards of the Roman administrative practices.

I would suppose that in Britannia something similar would happen, until the arrival of the Angles, Saxons and Jutes.

My doubt here is if we are not wrongly used to think in the Roman Empire looking always West (of Rome), and instead we should look more into the East to search for the print of Rome after her fall. And no, I'm not refering to just Byzantium.