Trading Post

 was a British special effects firm steadily active from roughly 1962 to 1969. It appears to have been run by two relatives, Bill King and Malcolm King. They were usually, but not always, mentioned during the Patrick Troughton era whenever Bill King was credited for special or visual effects design. In all, they appear on the credits of three season 6 serials: The Invasion, The Krotons and The Seeds of Death.

Their sudden unavailability after Seeds caused tensions between the Peter Bryant's production office and the BBC's Visual Effects Department. No longer able to rely on freelancer King, the VED were faced with much more work than they could handle, given the complex demands of both The Space Pirates and The War Games. This difficulty was solved by hiring John Wood, an ex-BBC designer who had gone freelance since his work on The Celestial Toymaker. (DWM 242)

Trading Post also hired out its stock of props, a facility of which early Doctor Who designers occasionally availed themselves. It's known, for instance, that Raymond Cusick got the interior spaceship panels and other set dressing for The Rescue from Trading Post. (REF: The First Doctor Handbook)