I watched all of the first series (Scott-Furlong Aircraft Ltd?)
I always remembered the aircraft as a bit of a Citation lookalike; didn't recall it was high wing!
I have a vague recollection that Handley Page built the mockup and the exteriors were filmed at Radlett. It was said at the time that for realism, HP actually designed it so it could potentially become a real aircraft, but there was no intention to actually build the real thing.
In a later series, they built the Scott-Furlong Predator VTOL aircraft which bore a startling resemblance to the contemporary Dassault 'Balzac' VTOL research aircraft; a sort of VTOL version of the Mirage 111 which if I recall correctly, was meant to be developed into the Mirage V operational aircraft.
No mention of actor Patrick Wymark in the credits but I'm sure he was in the series at some point.