ATC backup
I have been on a day visit to NATS and when I suggested what happens if their nice shiny glass roof comes down, I met a few grim faces and was told: "well we are all dispensable", and there is a backup almost next door... one can only guess what "next door" means... in any case they are not down, they have limited capacity, i suppose that a backup facility would have limited capacity as well, due to staff not normally working these sectors and possible same or lower coverage or both.