London to Athens is a major route. When I was a travel agent (here we go again...) that was probably the single route I booked the most seats. Whatever else happened each day, I always booked a dozen seats, probably 60% Olympic 40% BA (we had better fares with OA). BA have always used a mix of 757s and 767s.
As a rule of thumb, the main European routes BA deploy 767s on are: Brussels (one or two a day; mostly 757s), Nice, Frankfurt (see BRU), Athens and Cyprus. Paris was all 767s til the Chunnel opened.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz