Spangenhelm helmets, in fact most of the helmets, and specifically the Roman ones, together with mail or scale or lamellar armour with trousers and puttees on the legs, large round shield and long sword, gives a decent approximation of a Late Roman soldier. Augustan or Trajan era armour would look weird for the setting. The last possible use, probably old armour, of the classic Roman armour was the late third century. The Age of Crisis made it hard to get current gear.
The mission at Hadrian's Wall gives a nice Roman officer's helmet, although the sword looks unusual. Lundenwic or whatever in this game is a Saxon trading port with some ruins alongside. Probably a neo-Roman lord, a second Arthur could live in some of the castles that look like Roman fortresses or small towns with a small villa and arena.