Supply and demand. Airlines will sell at the fare they think they can. If BA is able to sell seats at that fare, good on them, and so is any other airline.

There is no rule saying fares must be affordable to anyone, let alone everyone

It is shout making a choice in life, whether that is Ryanair, Easeyjet or BA, or STN, LHR or LTN etc and picking what suits you best.

Personally, the cost of getting to STN by minicab far exceeds the cost of getting to LHR from near LHR when on multiple flights (cannot call them 'connecting!) that €180 (especially if that is a return!) would not make me choose STN, ever... but since I hate STN and LTN, I would pay much more to avoid those airports even if getting to/from does not cost more.

But everyone's situation is different and it's good that we have a choice!