In normal competitive business you can pass whatever costs you like onto your customer. That's the environment that NSL operates in and it's quite free to pass its pension costs on. Unfortunately that cosy arrangement only works as long as all your competitors do the same. As soon as one reduces its costs the others will usually have to follow suit or go out of business. Airports will pay a premium for the NATS service but there is a limit to that premium especially at the smaller airports.
NERL is effectively a monopoly where a whole different set of rules apply. I'm sure Paul Barron and any other CEO would love to be in charge of an unregulated company with a monopoly and just pass on whatever costs are incurred but it was never going to happen. The Government decided that NATS should be a regulated business and laid down in the Transport Act exactly how the Regulator should regulate. It also laid down an appeals process through the Competition Commission but gave that body very similar guidance. NATS does what it can to influence the regulator's decisions and invests a lot of effort in doing that but there is a limit to that influence however good your Chief Executive.
The Government PPP deal is at the root of all this but we lost that battle some time ago. We are where we are and no amount of dreaming will get us back. Even if NATS does return to the nationalised fold things will never be the same again.