Confidence: It's likely to be this, but I can't be certain.
Notes: Tapered tail, very translucent, and with ridges (fingerprint-like) on mantle centred on the midline rather than the breathing pore, all suggesting Lehmannia. There are no opaque white glands in the skin (visible in D. reticulatum in close-up). Deroceras are very rarely stripey, whereas yours has two dark stripes, in the right place for L. valentiana. The other option, in woods more than in gardens, is Tree Slug, L. marginata, which has weaker, usually lower, dark stripes, and usually has a whiteish stripe down the keel.