Confidence: It's likely to be this, but I can't be certain.
Notes: Certainly a large Arion. Khaki with mustard-coloured footfringe is very typical of A.flagellus. An opaque creamy sole, not darkened toward the edges, would increase the likelihood. The fairly wide-spaced tubercles on the back likewise suggest flagellus. Other option would be a pale A. vulgaris (=lusitanicus). Disssection would confirm. Unusually, juvenile flagellus are usually certainly ID'd on externals: well banded, with 9 or fewer tubercles between the dark bands.