Confidence: It's likely to be this, but I can't be certain.
Notes: Around 10cm long, seen evening (approx 7pm) in a small woodland amongst buildings on University of Sussex campus, Falmer (nr Brighton). It was heading for the carcass of another unfortunately squashed slug. It was the longest, fattest slug I have ever seen in my life (!) which was why it attracted my attention. The weather was warm and a little humid following rain earlier in the afternoon.