I was actually researching this myself last week, because it's a career path I'm somewhat interested in.
From what I can see, it shouldn't be a problem. I read several accounts of people who have worked or flown with transgender pilots and cabin crew, and there wasn't a problem. A few years ago there were some people who got fired over it, but in this day and age we're protected by law. BA seems to have a good record with transgender staff, info on the other airlines wasn't much, but it shouldn't stop you seeking employment with them.
They care about your physical fitness and people skills. I don't know if you'd have to tell your employers about your trans status for medical/documentation reasons, but there should be no real reason your co-workers would be informed. I'd say just ring or email a few of the major airline careers centres, and ask for their policy.