Adder????????
Dude, If I ever found an adder that big by me, I'd be wishing I'd brought a bigger snake hook.
OK, this one will have you. Brit species.
Dogfish or some kind of ray? (no not that Ray!)
Cheers,
Tom
Probably the thornback ray (Raja clavata) then although the starry skate looks quite similar
Edit: thornback ray it is! The properties of your image were very helpful
Cheers,
Tom
Cant even see which way round it is