Yeah I had a summer job there back in the stone age - a lot of the baggage vehicles (one was a Karrier Bantam) were unreg. - no tax (none needed) as they stayed 100% airside.
Also true for some tractors that never left the farm - in the olden days at least. My mate bought a 1950s grey Fergie in the late seventies and it got a T suffix plate when he registered it as it was the first DVLA had heard of it. These days it'd have an age related plate, but that was how it worked then - you used to see 50s Chevies with an S or T suffix as that's when they were first UK registered.