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Lots of pictures of this species of slug on http://www.seaslugforum.net/factsheet/hermbifi
The description there "white body with a rosy blush on the rhinophores and elsewhere on the head, the digestive gland showing through the skin red-brown". The colour tends to fade in unfed animals. There is a conspicuous brownish duct of the digestive gland down each side of the body giving off a branch to each cera" looks quite good in relation to your picture.
It does, doesn't it? Especially the fading colour - I've got that specimen and one where the colour really has faded badly and the whole thing has started to decompose. I just don't want to be optimistic that is it H. bifida without other opinions, because if it is H bifida, it would prove a nice first record for Kent / SE England (according to NBN data)!
According to Thompson this smells either of Herb Robert or Hydrogen Sulphide. Not commonly found on shore. Was it on a red alga? Sub-tidal or on shore?
They were on Halurus equisetifolius, on the very low shore, next to the kelp zone
Lat/Lng: 51.148, 1.3848
OS grid ref: TR368441