Orca, there are several reasons, many of which are listed amongst the 400+ replies to the DM thread, the 200+ comments on Arrse, e-goat etc. And she is a friend of a FB friend, and even through the medium of social media, she is yet to come up with a reason why she chose to spend the best part of 24 hours travelling in uniform when she didnt have to.
However the main one is JSP800, which details the class of travel and the mandatory rest time after a long-haul flight. I'm sure as an aviator, you would not be comfortable taking an aircraft that had been serviced by someone who had spent the last 24 hours travelling half-way across the world to be rushed to join "your fight".
I doubt "But I looked smart in my MTP sir" would wash in any subsequent BOI.