Hawker. Where would the RAF have been between the wars without all those biplanes (maybe we should have had the Fairey Fox instead of some though) Or in the BoB withouth the Hurricane? Or postwar without the Hunter, Seahawk & Harrier?

de Havilland. Almost single-handedly founded the light aircraft movement worldwide with the Moth. So many classic types and so much innovation in airframes, engines, propellors and missiles.

Bristol. Like DH, so many important aircraft, engines and missiles for so many years, although not nearly as “glamorous” as DH.

Handley Page. As Cees so rightly points out, the RAF was never without an HP type until the demise of the Victor (or maybe Jetstream)

Shorts. First registered a/c company in the world I think, and still operating.