There is a huge risk on BA that the answer once airbourne will be a big 'no'. If you are able to negotiate for it, the answer will be one time and for a limited amount of time. No swapping for meals. I guess what I am trying to say is 'don't count on it'. Most especially if there is a fancy VIP in First. Then the answer will be 'no'.

I've watched a lot of people try. On one flight with a royal princess in First we weren't even allowed to leave the cabin for a walk! It was stay here and stay put!