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Its a goby but they are a group that I find difficult to seperate into species and although Ive seen a few in recent years I'm not sure of the amount of variability in colouration even in a single species.
A fish expert on the British Marine Life Society page on facebook said they are a hard group to seperate into species sometimes even in the hand never mind from photos.
All I can say is its a goby and not a blenny.
It doesnt appear to have the frilly structures immediately behind the pectoral fins typical of the Gobius genus, if I'm right about that(ie the absence of frills) then its likely to be Pomatoschistus genus eg Pomatoschistus minutus(Sand Goby) or Pomatoschistus microps(Common Goby). There may be other possibilities.
...Goby. Could be any, need to remove from rockpool for clearer ID.
Best guess: Common Goby Pomatoschistus microps
Excellent comments on Gobies, with which I fully agree. I will just add that from my experience of gobies (daily in my aquarium as well as on the shore), Rock Gobies vary enormously in colouration and markings, but have more of a 'bull-head' appearance from above. But you do need to see much more detail than is possible in these pics, to note eg whether there are free rays above the pectorals or a yellow band along the top of the first dorsal. So I will say that I am convinced it is not a Rock Goby, and would endorse suggestions of either Common or Sand as being good possibilities.
Lat/Lng: 51.7238, -4.6843
OS grid ref: SN146061