To be fair True Blue I do take the point, I travelled overnight to Manchester for work recently out Tuesday back Wednesday. I couldn't actually use FR despite being cheaper and used EIR. The problem being FR only had 1 early morning flight both days. I could have used U2 from BFS but it's almost half the journey time at the time of day to BHD and with work paying, EIR won.
On the point of being flexible I guess it depends where you need to be in London and when but rail tickets are priced in singles a lot in England and it can be a case of arrival in LTN and return from STN for example, and not necessarily the same airline!