I don't know how things in the UK work with portion sizes, but every buffet I've ever been to where we pay for head counts there is far and away a good amount left over for people to snag seconds and even thirds if they wanted to.
I will definitely find out before confirming we go to this method of eating, but to be clear, I am not down with us going to a bar/pub/restaurant and then dealing with fifty different people in our party ordering separate checks and having it all be a huge mess like that and worrying about space and everything. If we're all still hungry after the event, let's go stuff our faces with appetizers outside the event site! I really and truly think that having a mass food order will make everything easier for everyone.