An interesting topic that really belongs in the Aviation History & Nostalgia section of PPRuNe - I'm sure a Mod will assist in moving it if necessary.
When began my spotting days BHX/LHR was a 2/3 times daily BEA Viscount service, with Vanguards on some late evening night stopping services.
I can't recall BHX/LGW operating before the BCAL PA31 Navajo service that began, if my memory isn't failing me, in the mid to late 1970s.