Numb3rs, there are many varieties of people who can do so, and reasons why they do so. The bulk will be on business although a few well organised OAP's with flexibility of travel dates (see above
) are leisure fliers. I guess since you are London based you are referring to personal travel? It is nearly two years since I last took a leisure flight (CW redemption).
I am usually flying weekly on business travel claiming expenses back from my company and they in turn from a client (I make my own bookings), one client requires fully flex economy bookings and will pay BA prices. Usually last minute.
If I want to upgrade cabin I can pay the difference over what the client pays, my company allows me to travel whatever route and class I deem appropriate. Much depends on where I am going and what I have to do there, my time billing clock starts when I set off. Having me hanging around with the clock running can turn out to be very expensive - and my work rates are relatively cheap! Billable hours can skew all sorts of calculations.
A long haul overnight flight with a sleep means I can work next day. That can be cheaper overall than getting me there the day before and putting me in a hotel overnight. Then returning me back home afterwards. Others may have a particular class of travel written into their terms of engagement.
As to cost, your combined income after tax, for example, comes to about my credit card spend on business travel in an average year. That usually pays for some premium cabins, but not normally with BA because they are very expensive. I use BA CE in Europe, but mainly EK and EY to/from Africa for cost & comfort. I get the avios etc from the flights and credit card spend for my private travel if I want but I seldom travel for leisure.
I have even done exEU for business as the cost justified the extra overnight travel (office bill rather than client bill), AMS-LHR-YYC return was the same price for two of us in CW as one going LHR-YYC.
No easy answer to your question, but in the main it is because somebody else is paying
