I completely agree that everyone should wear the hat. The BA uniform is beautiful and the hat definitely delivers a very professional and somewhat 'british look' to the corporate identity of British Airways.
At bmi there are very strict rules about wearing the hat for both cabin crew and ground staff. The idea is that the public must not be able to distinguish the difference between ground staff and cabin crew (apart from the wings) as both work groups should meet the airline strict uniform standards, after all they are all important customer facing frontline staff! I know the bmi uniform is well out dated but the company still try their best to get the staff the wear the uniform to a very high standard with pride and the hat helps complete this trick!