In message , at 18:58:17 on
Sun, 2 Apr 2017, Graham Harrison
remarked:
It could even be cheaper. When I was booking London/San Francisco
return last year a chance remark had me look at Manchester/London/San
Francisco. That was cheaper even though I was using the same flights
LON/SFO/LON. Aren't market forces wonderful?!

That's because you were in fact paying the "right" price, and people
paying more were mainly doing so for the perceived benefit of both a
direct flight and easier access (for them) because it's Heathrow.

Market forces do play a small part in that the airline would prefer to
sell the lucrative trans-atlantic leg to a passenger they subsidise on
the domestic leg, rather than have them fly from Manchester direct to
the USA on different airline, leaving them an empty seat.

There's also a certain amount of "no-one got fired buying a direct
ticket on a well known airline from Heathrow" effect.
--
Roland Perry