I've reported my sighting of "Blue KKS" to The Severn Estuary Gull Group (SEGG).
No interactions present.
if you get anything back from SEGG.
The bird has the round head feature of argenteus but I think a view of the wings and measurement would be useful features in being sure.
Lat/Lng: 51.2105, -4.1221
OS grid ref: SS518478