My guess is that this treasure is indeed a King’s Treasure - possibly the treasure of the Kings of Northumbria from about 650 AD.
Reason: if I have learned anything at all from years of playing “Medieval Total War”, it’s that the Northumbrians were Christian and well-lettered before anyone else in England. Whereas Penda of Mercia (where the treasure was found) was a Pagan.
If I had to guess I’d say this stuff was once the regalia of Edwin of Northumbria.