Slugs

Arion (Arion) rufus

Arion (Kobeltia) distinctus

Limacus maculatus

Deroceras (Deroceras) reticulatum
Snails

Cornu aspersum

Monacha (Monacha) cantiana

Oxychilus (Oxychilus) navarricus

Clausilia (Clausilia) bidentata

Ampullaceana balthica
Freshwater Bivalves
Many thanks to Ben Rowson and Chris du Feu from iRecord who helped confirm and correct species where appropriate.
Useful AIDGAP reference books:
Slugs of Britain and Ireland by Ben Rowson, James Turner, Roy Anderson and Bill Symondson.
Land Snails in the British Isles by Robert Cameron.
Freshwater Bivalves of Britain and Ireland by Ian Killeen, David Aldridge and Graham Oliver.