In the report on the BBC website on this event, it was said that the ATC systems were operating at 98%, or 99%, of capacity. That is surely most of the problem. If stuff happens--and stuff happens, it's an axiom--it's much easier to cope with if you have spare capacity.
And why don't we have spare capacity, in all sorts of systems? Cost reduction, of course. So if, and only if, the head of NATS is responsible for running the system at maximum capacity, he should resign. But I expect it's the paymasters who really are responsible, and the fact that it's a public-private partnership, one of the advantages of which is private sector financial discipline. That is, running everything at maximum capacity all the time. Why have such big engines? Run them at take off power all the time. Why plan for engine failure? Plan for success.