This is what
Sear says in RCVM:
"There was further unrest in
Britain in the mid-150s, the beginning of an unsettled period in the island province which saw the abandonment of the Antonine Wall between the Forth and Clyde and the recommissioning of the more southerly Hadrianic frontier.
The careless striking of this
type and the fact that they almost never turn up outside of
Britain may indicate that they were struck in the province, perhaps in the region of the northern frontier."
I don't think it would have required the presence of the Emperor to
mint coins. Setting up a temporary
mint wouldn't have been that difficult.