BEWARE! There is only one species of crayfish which can legally be sold and kept as a pet in the UK at present...the red-clawed crayfish (distinguished by having a red patch on the outside of its claw).
Please take a look at Sheffield City Council's website about crayfish...
sheffield.gov.uk/out--about/parks-woodlands--countryside/ecology-service/biodiversity-action-plans/crayfish-conservation
Our native crayfish and other wildlife are suffering from the release of pet crayfish into the wild, make sure you don't add to this. Make sure you dispose of any unwanted pets sensibly, apparently you can even contact the Environment Agency if you want to dispose of them humainly and safely.
Make sure the pet shop are aware of this too.