Not everybody in the UK has an exhaustive command of regional accents, including people who work on BBC dramas. It's very possible they just made something up and hoped to get away with it.
I suppose it's possible, but the accents otherwise seemed very natural to that area and to the actors, and it seems remarkable as a choice of invention.
I'm reminded of "I can't use contractions" Cmdr Data in Star Trek. (He does, all the time). It didn't seem an artificial choice in that way.
