England was nowhere near 90%, or even 50%, forested, even before the Romans arrived. Hot “baths” weren’t unknown in the Anglo-Saxon period, and they’re also recorded in the Icelandic sagas amongst the vikings. One so hot a man killed a berserker by tricking him into the bath and giving him fatal heat stroke. There’s no reason to believe landowners left en masse after 408.
What they did do is stop building in stone. One of the most vexing problems is that there is basically no trace of Britons during this period. Some archaeologists posit a kind of de-Romanization, in which Britons self-consciously abandoned cities, building in stone, mass-produced pottery, and other characteristic material forms of Roman life. Rees’ My Roman Britain is a recent example.
Oh, it was all very helpful! I had to cut my trip short, but I had a wonderful time. Saw three Roman sites, three castles, and met dozens of great people. I’m hoping to come back sometime with my family!