Confidence: It's likely to be this, but I can't be certain.
Notes: The two dark bands along the edges of the mantle and down the sides, the breathing pore in the front half of the mantle (the right mantle band arches up over it), and the lack of a keel make it an Arion, a round-back slug. The colour and the footfringe hidden by the slightly-bulging sides of the body make it a Garden Slug, A. hortensis agg. It's difficult to be sure which of the three species in the group it belongs to; the high right mantle band with no hint of a dip or break in the upper edge suggests hortensis s.s. as would reddish colour in the tentacles (but it might be brick showing through!).