I posted some years ago about a UK Lord something or another (a Premier) who changed right in the middle of the F cabin. It was hugely embarrassing for all concerned, except the senile old git himself. I would strongly recommend the bog as mentioned.
There is something to be said for waiting until the wheels are up, as the likelihood of needing to make a rapid elegant exit diminishes after that point. It is also worth keeping shoes on until that moment as well.
Found it!
http://www.flyertalk.com/forum/briti...riate-not.html