The corporal when in his battle dress wears his webbing belt upside down. No corporal would ever do that.
The RAF Policeman on the gate in the credits is shown wearing white webbing and a holster but his cross strap is shown on the same side as the holster, looped over his left shoulder instead of being a cross strap across his body.
The car used to bring Matthew Lilley to the camp, a white Morris Minor, bears a
registration number in black characters on a white background. This type of plate was introduced in the United Kingdom in 1969, although the programme titles date the episode as "1955". The car should have borne black plates with white [or silver/grey] lettering at that time.
The opening titles show an RAF Policeman in a sentry box. He is a sergeant who didn't carry out such duties and he is wearing his cross strap in his webbing over the wrong shoulder. In fact it doesn't cross his body at all.