When the Vampire aircraft is getting ready to start the dummy run, it is clearly below the cloud cover as the landscape underneath is very clear and without clouds. When it starts the dive, it is above a solid cloud cover and has to break through it.
In the briefing room, Kieron Moore says to the pilots that they'd all 'had your dummy runs'. But at that point, he was the only one who had had a dummy run (when his aircraft was damaged).
When the damaged Vampire aircraft is about to land, there would always be a fire truck and ambulance at the end of the runway. Neither of these is present in the film.
In the opening scene and several others, a black-headed gull is (wrongly) shown making the sound of a large gull species. A bearded tit is shown making sounds they don't make. A flock of ducks is shown calling like wigeon, when there are no flocks of wigeon in Norfolk in summer.
When Bill is strapping the Sqn. Ldr. into his Vampire, the boom mic is glaringly visible reflected in the canopy glass.
Communication with the firing range was via a flimsy telephone wire. In real such critical circumstances, this would have been via radio comms.
Sally says there are no birds breeding at that time of the year when it's obviously breeding season for most birds.
The tame (black-headed) gull 'Perdita' is said by Sally to be female. It's impossible to sex gulls on their plumage. As it's an immature, one couldn't even sex it by its breeding behaviour.
Bill says: 'Such a little island, isn't it... I was thinking of England.' He meant Britain.