On the subject of non standard BA747s I seem to remember they had one that had odd colours in the late 1970s -being all white witha blue cheat line rather than the normal blue lower fuselage.
I lived in Ashford at the time (the Middlesex one) and it often used to head off via Dover departure and have a vague memeory that soemone told me they only operated to Saudi.
Anyone able to fill in more detail, it seemed to me (and as I grew up right next to LHR I was pretty sensitive to aircraft sounds ) to make a slightly different noise but as BA 136s had PW engines too thats probably wrong)
PB