It is a panonnian hat. There is of course no proof of its use in Britain as late as the 580's but the Britons certainly look to their roman past especially for all things military. We know they used the draco and we also know from Bede the Bernicians used another enseign known to them as the thuf and to the Romans as the thuffa. The Sutton Hoo royal burial show us a 7th century king who probably fancied himself as a roman officer even with heavy germanic fashions (or the other way round). Both Britons and Bernicians garrisoned some forts of the Hadrian Wall by the time. Why not keeping a popular roman hat? And the form, fairly basic, may have been known for other style of hat...