NigelonDraft, that third strip may seem trivial us, but because BA recruits so many cadet pilots, having 2 stripes will tend to "represent" less experience as that is what new starter(usually a cadet) wears.
Why not just let those that joined from other airlines or organisations, with several thousand hours of experience wear a third stripe. The status of the extra stripe represents flying experience well inexcess to the cadet entry. That is what stripes really represent - experience. Doesn't matter where it comes from, as long as it is relevant!
As it is a trivial matter, then just let it be done and keep everyone happy!
But I fear the cadet entry 2 stripers won't like it...!