I would think it was in the morning or early afternoon, definitely well before they open. I suspect it was very shortly after the segment where he was in the kitchen since it was obvious that was not a full staff in the kitchen and when they were eating the camera was positioned where you could not see the food preparation area except reflected in the window/mirror. They probably only prepared the two dishes that they showed being served. I seriously doubt that V&A would let a camera crew interfere with their service.
I think they also taped the meal at CG before it re-opened. It was way too quiet while they were eating there and the people at the bar and walking around looked too much like staff members than people actually eating there.