This rather unpleasant looking fish is a hagfish Myxine glutinosa, a member of a very ancient order of jawless fish. They live in muddy sediments and feed on dead and dying fish, sometimes burrowing through their flesh and consuming the fish from the inside out. They've also been found inside of human corpses that have been in the sea for a long time.
I've twice found these unlovable fish on the shore, alive, along the North East coast. The first occasion was at Whitburn Rocks in Tyne and Wear (see pictures at the bottom of this post) and this one was on the Alnmouth end of Warkworth beach in Northumberland. Both locations were close to river mouths and I suspect that had been thrown overboard by fishermen cleaning their catches on their way into port.
Hagfish are blind, although they can perceive light, and they find their prey by scent, using the barbels on their nose. Aside from their feeding habits, their most unpleasant characteristic is that they release massive amounts of slime when they are caught - you can watch this here on YouTube. This is sufficient to deter an attacking shark - see this YouTube video. Even more remarkably, these fish can tie themselves in knots because they have no rigid vertebral column.
This is the mouth, with tooth-like projections inside that the hagfish uses to chew a hole in the body wall of dead fish. That 'knot-tying' behaviour allows it to twist and turn, tearing a hole in its prey.
These are old pictures - I took them in 2007.
You can read more about hagfish here.