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Could be Coryphella browni or Coryphella lineata, which have similar white bodies, long front tentacles and cerata with white tips, but theirs are also on the back. I think Facelina bostoniensis has a sort of bald patch along its back, because the cerata are in two rows along the sides of the body.
Or is this the same species re-named?
The differences are shown in marlin.ac.uk & habitas.org.uk. Both coryphellas are thin "browni has a narrow foot" (marlin) & "lineata is a slender species"; Lineata has conspicuous white lines on body which would show on my pic (between the rhinophores). In contrast bostonensis is broad "a much broader body than Facelina auriculata" (habitas) & "foot broad" (habitas)
bostonensis has much longer cerata "cerata up to 1/3 or 1/2 body length" (marlin) "cerata long,overlapping" (hab). +neither coryphella shows the red oesophagus that facelinas show (behind the rhinophores).
I also noticed the colour in bostonensis cerata is more of a thin line "visible through the cerata epidermis", but in coryphellas a solid colour.
eg http://www.nudibranch.org/Scottish%20Nudibranchs/html/facelina-bostonien... , http://www.marlin.ac.uk/speciesinformation.php?speciesID=4645
Size about 2cm
Lat/Lng: 51.5607, -4.3131
OS grid ref: SS397871