This nudibranch, Polycera faroensis, is very similar to the one in the next picture (Polycera quadralineata) but tends to be slightly longer (up to 5cms) and thicker. Its yellow markings also tend to be a stronger colour. Polycera faroensis always has at least eight yellow or yellow tipped feeler-like projections in the oral area. It feeds mainly on bryozoans in shallow water, particularly on steep walls. In my experience, they are often seen in pairs. Found on most UK coasts, except sandy and silty areas.
St. Abbs Marine Reserve, Scotland. North Sea