how about wearing it, at least until you board then change??
but seriously.....i'm not sure exactly what the rules are but from a crew perspective we will always try to accommodate bringing it on board as I'm sure most of my ground colleagues would be happy to allow it.
We do get them onboard fairly often and it is obviously a great opportunity to engage with crew/passengers. going to BDA there should be plenty of space in the wardrobes of the 777, sometimes more of a problem on SH but we normally manage.
Best wishes for the big day
JJ
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views expressed here are those of my own and do not necessarily reflect those of my employer British Airways