Nice coin. You get quite a lot of Postumus antoniniani from Cologne found in Britain.
Often his coins are described as 'Cologne or Trier', because I don't think anyone has worked out whether he had mints at both Cologne and Trier (which was his capital) or just one of them. And if both, the coins don't often have mintmarks to tell you which, so you have to go by style. Lyons (Lugdunum) and Milan are also possibilities but apparently their style is easier to tell apart (although not by me!)