urok - what's a 'hostie'?
As the question was based on a BA 3 class 777 operating to KIN, I can say, with reasonable confidence that the wardrobe to passenger ratio will allow the crew to stash the odd wedding dress.
Unless you are a high value Gold Executive Club member, BA don't really have a pre-travel service where you can prepare for such a thing. Anyone you speak to anyone on the phone will just tell you check it. From my experience in customer relations, dealing with baggage issues, that is probably the last thing you should do with it - apart from letting the groom wear it.
Bring it on board, you will be fine.