"The Register approached NATS for comment on how the software was purchased and validated."
According to some of the aviation-related websites I've looked at, the core software was purchased the U.S. Federal Aviation Administration in the 1970s - yes fifty years ago - and currently runs in emulation mode on more recent, but by no mans new, hardware. I have even heard that it runs in a virtual image of an older model mainframe, which is in turn emulating the original hardware. The software has of course been heavily patched, but there's nobody still working who was there at the beginning and who knows how it all actually works, they've all long retired and/or shuffled off their mortal coil.
The only solution is a complete rip-and-replace, but since NATS has been sort-of privatised, that prospect brings on an attack of the cold sweats in the beanie department and the Board.
Mind you, I don't blame them, this would probably be a Major IT Procurement Project with a huge budget, maximum visibility, long timescales, many many stakeholders, a lot of politics and enormous technical risk.
There are times when I'm glad I'm retired :)