I'm pretty sure the reason the Vanguard's First Class cabin was at the rear was because it was there on all propliners, due to it being rather quieter behind the prop's, instead of in line with or ahead of them. Reverse applies to jets.
We've been discussing this recently on another thread. Mainline Dash 8s (Austrian, Croatia etc) have their business class at the (noisier) front end - because passengers expect that's where business class should be.
Some of this however could perhaps be put down to much busier air routes nowadays.
These are sectors
into LHR - include contingency for holding in the stack, presumably.