Oh, I forgot, Calumet will defintely have film, but not an on-site lab. Plus they have stores everywhere in the UK.
I've never been to a Jessops, but if it's like a Ritz Camera or Black's in Canada, I'd forget about them. Those stores are geared to the general consumer and not hobbiest or professionals.
The labs and stores I listed will definitely have your films and develop it (professionally in the case of the labs) as well.