We're in the middle of replacing a section of floor in the old farmhouse at Homerton and found this resident beneath the floorboards.
I believe it is the large introduced Leopard Snail Limax maximus. I now have the correct identification. It's the introduced nocturnal Yellow Slug Limacus (Limax) flavus. See comments. (Thanks Snail.) This one was about 6 cm long. We used to find slug slime tracks across the carpet so hopefully the new floor will keep this creature outside.