The discussions in the following threads contain much
in-depth information (plus photos of seldom encountered
coins) relating to Britannic coinage and the Tetrarchy.
If you have a deep interest in the background of many
of the recent developments in the study of this
coinage, you may enjoy reading these subjects:
Constantius' Boulogne mint
Maximianus Herculius, AE Follis - London or Continental Mint?
Unmarked London Mint Type
Invasion coinage of Constantius