Ive had a trawl through a few id websites and it certainly looks like its a Grey Gurnard Al
The clear spiny lateral line is what is leading me to this conclusion along with the colour even though as you say, it doesnt have the mark on the dorsal fin.
Its certainly not a Red or a Tub, and i cant match it to either the Streaked Gurnard, Piper Gurnard or the Long Finned Gurnard, all of which can be caught in the uk but are coloured more like Tubs and Reds.
More info available on this link http://britishseafishing.co.uk/gurnard/
I have also googled pictures of all 3 rarer types and it doesnt match any of them.
Sorry!