The status of Luffia species in Britain has been confused for some time. The checklist and most books list two species; L. lapidella which has a wingless female but fully winged male and is only known from Cornwall, and L. ferchaultella for which only the wingless parthenogenetic female is known.
My understanding is that recent genetic research has shown that these are in fact a single species and the name lapidella has precedence but until this is published in the forthcoming new checklist or elsewhere, I guess we should stick with calling the form found away from Cornwall L. ferchaultella.
Whatever its taxanomic status, Luffia cases can be found on tree trunks and occasionally other wood or rock. It seems to be particularly common on oak. The cases are only around 5mm long and are covered in algae so are quite hard to see.