Thanks to both!

I'm genuinely intrigued why BA (and to be fair some other major airlines) seem to make it so difficult to browse flight options.

I just confirmed my recollections from EasyJet - look on their website, and you first get a drop down menu with all departure airports - select one of those and then you get a drop down menu with all the possible destinations from that airport. Easy(!) and intuitive - should be a lesson BA et al.