Thank you kindly!
Technically, yes it was. There are two explanations, in-universe and out, as to why this is not the case here.One thing I don't quite get is why Britain is so important at this stage in your timeline? My knowledge of that period is really hazy, but weren't the British Isles a backwater at this point in history?
In-universe: there was always some potential for Britain (and Gaul) to break away as their own empires, just look at the Gallic Empire in the early 270s. The Romans were basically compelled to vacate the island in the early 400s OTL to fight against the various tribes moving westward at an alarming rate, and with most of the forces gone the few remaining folks fell easily to the Anglo-Saxons to the east, Picts to the north, and Goidelics to the west. (The first of these, not least, was because some bright sparks insisted that bringing in a warrior race would absolutely offer protection against other warrior races.) In this particular universe, crossing the Channel has become immensely difficult in recent years, thanks to that eruption over in Indonesia causing massive storms to rush in from the Atlantic. This has a two-fold effect: it makes Britain more isolated (the troops have a harder time leaving and a harder time coming home, and by gum are they more inclined to come home again than leave again) and it decimates the Germanic tribes who live near the coasts. The Franks are fine (for now), but the Angles and Saxons? Poor folks had to regroup, join up with the Frisians and settle down in northern *Germany, at least for now. Meanwhile, those warriors who still make the suicidal attempt to cross are stopped by the reinvigorated "Roman" troops, along with newly-introduced Goidelic tribes, given land in the south of Britannia Prima (i.e. Wales) in exchange for protection along the Channel. So the Isles have protection, they have an army, and they have an emperor--a decent place for cultured folk to escape after the fall of the Empire in Gaul (which is why more folks than not speak Gothisque instead of *French).
Out-of-universe: this was originally concocted as a suggestion for a colleague of mine on DeviantArt, who wanted to try experimenting with worlds where the Insular Celts, Haudenosaunee League, and Japanese Empire all had their heydays. My idea was, here's a very simple geological/meteorological device that could possibly guarantee all three happening at the same time...