I think you will find there was no sickness. Just not enough controllers with the correct validations. I believe management were hoping to overcome this problem by bringing in overtimers but nobody volunteered. Why ? because they are trying to do it on the cheap.
NATS idea of good fiscal management is to pay the controller peanuts for overtime thus saving a few hundred quid and pass the burden onto the airlines as they can collectively afford the tens of thousands pounds extra cost of long reroutes and disrupted schedules.