Unidentified Sea Slug? Sidmouth, Devon

Unidentified Sea Slug? Sidmouth, Devon

Observation - Unidentified Sea Slug? Sidmouth, Devon - UK and Ireland. Description: Individual observed on underside of groin boulder along Sidmouth beach at low tide. Approx. 12mm length (5 pence size reference) with rounded appearance and warty skin. I believe this is a nudibranch species and does look similar to Pleurobranchus membra

Individual observed on underside of groin boulder along Sidmouth beach at low tide. Approx. 12mm length (5 pence size reference) with rounded appearance and warty skin. I believe this is a nudibranch species and does look similar to Pleurobranchus membranaceus but I have really struggled with this ID so would really appreciate some help? For example, what is the horseshoe shaped appendage at the front?