It's a lunch sufficiently nice that the whole day is taken up.
I've been out on these a couple of times, but I'm not actually on the one occurring today, for which a rather nice venue, Gordon Ramsay au Trianon, has been chosen. If you examine that name closely, you'll notice that part of it is French and, indeed, so is the restaurant, being Ramsay's first in France.
The plan was for people to meet up at the Champagne bar in St Pancras and to travel first class on the Eurostar there and back.
I wonder what the fall-back plan will be, given that the tunnel is closed.