There was a story I heard immediately post 9/11 that one plane had a load of Marines in uniform on board, when it landed at nearest airport as instructed, a number of young men amongst the passengers quietly disappeared. probably not true, but I think the presence of uniformed personnel is going to make a bad guy think twice.
On wearing uniform in UK, RN does appear to be different - RN were allowed to wear uniform on last weekend's Gay something march, RAF and Army were not, although there was of course no ban on attending