I'm sorry to have to pop a few balloons on this one...

It is down to one thing, and one thing only... BA does not properly ensure that their systems are tested and integrated when one part is upgraded - whether that be "front end" (what you as a user see) or "back end" (how your actions are translated into bookings and changes to these). For years it has been a problem that BA insist on using a consumer facing online advertising agency/web development company to develop their front end while insisting that a traditional B2B systems integrator handles their back end development. This has never been an easy marriage and I can think of numerous times when the front end has changed, without the back end being able to follow it. It happened to me as recently as last night when I tried to pay the outstanding amount on a holiday. I got an error message and had no luck despite repeated attempts over the course of 4 hours. Back in August/September 2012 I had to book about 6 flights through alternate carriers or book BA through Expedia as the front end wasn't communicating properly with the back end.

I applaud the fact that BA are trying to ensure a smoother user experience by employing 3rd parties who know ho9w users want to interact with the airline in the booking situation, but there is still a lot of work to be done when it comes to making sure the process actually runs smoothly...

A few years ago (probably almost a decade by now), Air France gave the responsibility of building their web site to a (primarily) back end developer. Even though the user experience was significantly lacking there were never any glitches between the front and back end integrations and BA seriously have to think about where they go from here. My personal recommendation would be that they allow a systems integrator (back end developer) to take over the task - with the key proviso that they spend x on hiring in the right people to ensure that the user experience also is at the level where it needs to be.

Also, don't get me started on the last two major re-designs of the homepage - it has actually gotten worse TWICE.