eel like around 30cm, grey coloured, dead, thought to have been dropped by gull
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I think it is an eel!
Certainly the overall elongated shape suggests an eel, but even as juveniles/elvers, members of the eel family have a dorsal fin running along the back around the end of the tail, then continuing beneath as the shorter anal fin (no breaks). I can't see this in the photo - if it was present (maybe damaged/folded flat - could the original sender confirm this? Also, can you remember if it was silvery/pale underneath?) then it could be an eel, but if not (e.g. if there is a break between the tail fin and dorsal/anal fins) it is something else. Not yet sure what that something might be though... if it was found dead, it could be discarded bycatch from a trawler (even if subsequently dropped by a gull...) and so be a deeper-water species.
It was certainly eel like but i really don't think it was an eel, and neither did my lecturer and yes it was pale underneath, i think you are right in suggesting a possible deep-water species....
I have a thought - see suggested identification...
Lat/Lng: 50.4, -5.1
OS grid ref: SW8060