I think there have been a few threads on the AA board about this as some flights Y passengers must use the Y lavs, and F/J must use their lavs, usually quoting some FAA rule. It was rather erractic though, and I think more on international hops. No idea about BA's policy, I use the loos in the cabin I am in due to those experiences on AA, but other than that have no idea why, I just made the assumption that is the way it was....and at risk of derailing the thread, I suppose the overhead bins (well on AA anyway, I assume so on BA too) are for everyone's use, why not the lavs?