When lifting a pot to plant out today, I found this Glass Snail. There are four British species, and I think this is Oxychilus draparnaudi, given its relatively large size (a mighty 15 mm). Glass snails are named after their shiny, fragile and translucent shells when alive. After weathering the shells rapidly lose their lustre. Oxychilus shells are rather flat, which makes it easier for them to hide under pots or stones.
Barker, G.M. (2004). Natural enemies of terrestrial molluscs. CABI. DOI: 10.1079/9780851993195.0279. Here.
Karin Mahlfeld, Karin. Impact of introduced gastropods on molluscan communities, northern North Island. here.