Nats have a major contingency provision for a Swanwick failure but its never going to be invoked for a problem of this scale.
It's possible their contingency is build for different scenarios, but based on the info we were given - it was the failure of a single system. Until I'm told otherwise.
I'm sure there is redundancy built in the telephone infrastructure, but I'm still surprised that this brought NATS down in such a way.
Swanwick has a major training facility, virtually next door to the main site that can be switched to take control of the live environment. If that was deemed appropriate, I'm sure they would have done so.
I worked on the business continuity plan for a major call centre. During the process I was staggered to find that the UK is littered with fully functioning, but vacant call centres. Desks, PC's the lot... People throw a lot of money at this.
I'm not comparing a call centre to NATS with their bespoke kit. But the principles are the same. Diverse routing, multiple pathways, redundancy... all built in from the start.