I am afraid I dont know much about the history of it-(it was from and to LHR)
or which tour operator it was for. I certainly remember Balkan in later years doing flights through most of the UK regional airports with 154s and I think 134s.
I can recall-we were delayed on the ground for some forgotten reason and that the Chief steward made an apology on the P.A and added some slightly insensitive comment along the lines of "I bet you cant all wait to get away from this god forsaken country" (cant recall the actual words-but I am sure you get the gist of it.)
He was then very embarrassed to have to publicly apologise again a short while later-as it turned out that amongst the passengers was some bigwig from the Bulgarian ministry of Tourism-so I suspect it may have been a special charter of some sort-possibly carrying travel agents?