Mainly grey body
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Not really certain from this photo/angle... Lehmannia valentiana could also be a possible.
Brian Cambridgeshire said:
"The weak keel, strong dark stripes, pinkish background, and rather watery body are all typical of Spanish Slug. It's become quite widespread in gardens in the last 20 years, having previously been confined to heated greenhouses."
It could be this but a juvenile Limax maximus seems more likely to me.
Lat/Lng: 51.4, -3.5
OS grid ref: ST0168