Geez, guys...
There's so many fantasy influences, and mixes of tech, that one can't put it in any real historical context.
Examples:
The Plate armor (As opposed to true Plate mail, which was armor plates backed with chain) is from around the 1400s.
The viking swords are something intermediate between a Gladius and a broadsword, and are ca. 1000; most of the historically-based weapons that weren't universal designs (clubs, spears, daggers, etc.) come from time periods ranging from the late Roman empire to the Renaissance, most of which weren't in common use for more than 200 years of that span.
the various helmet designs range from ca. 600 to 1400
Most of the SE weapons are from the 1500s Japan or from much earlier Chinese sources that were rare by the time the Japanese items rolled around.
As for time settings as an alternate Ultima setting -
UO could be seen as a highly variant alternate setting, starting roughly in the time period of Ultima III, but with huge chunks of the world missing (for example, the ruined city at the Chaos Moongate in Ilshenar, was, in Ultima III, a city on the south coast of the same continent as Britain, divided by a river)
Magincia was destroyed in the time period around Ultima IV, and the current events correspond in ways, to the events of Ultima V - with Stygian Abyss corresponding to Ultima Underworld.
One BIG difference is that there were several races in Ultima up to U:IV, whereas the Sosarian reflections of the original Britannia in UO were human-only, UNTIL almost the same time period that the other races disappeared in the solo games. Coincidence???