No interactions present.
I just had a look at the NBn map out of interest. The highest frequency of records for turbot just happens to be exactly where you saw this little flatfish, around the Pembrokeshire coast and eastwards into the Bristol Channel.
It looks exactly as a turbot should, as broad as it is long and beautifully patterned on the back, perfectly camouflaged against the sand on which it is resting. I'm less familiar with brill images and it is a similar species but Im 99% sure your little fish is a turbot. Small turbot such as this are frequently caught by beach anglers pursuing bass and codling, such images are commonplace on angling forums whereas brill are extremely rare- not sure Ive ever seen one come up. Its a lovely little fish, pity it is injured.
..Brill - has small frilly edge to dorsal fin. Not Flounder has square cut tail fin. Left-eyed flatfish and round tail fin - this is a turbot.
Thanks for all the comments. My first thought was Turbot and I then talked myself out of it.
feeling - you can only be wrong - as I am on loads of occasions but it's the only way we learn.
Lat/Lng: 51.6782, -4.7004
OS grid ref: SN133011