Well, there has been a few actors that have appeared in #DoctorWho that would later become main characters in #KeepingUpAppearances
Clive Swift (Richard Bucket): Jobel (Revelation of the Daleks) and Mr. Cooper (Voyage of the Damned)
Geoffrey Hughes (Onslow): Mr. Popplewick (The Ultimate Foe)
Judy Cornwell (Daisy): Maddy (ep. Paradise)