Is the registration plate AST 281 in the Emmerdale garage genuine and if so how was it acquired?

no

For liability reasons, most film/tv cars have phony plates, unless they are registered to the studio or belong to someone related to the show-not concerned about his/her privacy. For example, there was a chase scene in an early Mission Impossible, where they "borrowed" Lucille Ball's Cadillac. You can always write ITC about the car in question.

It is not a registration plate. It's a trade plate and would have been (if real) issued many years ago to a garage in Inverness-shire. The DVLA will be able to confirm whether that mark was ever actually issued.

AST 281 was a blue Standard 8 owned by my father in Inverness from early '40's to 1946. We later parked next to it in an Edinburgh carpark in early 1950's.