If NATS really does want to keep airport contracts then it must bite the bullet and reverse the damaging and crass decision to split NATS into two and calling them NSL and NERL.
It wasn't a NATS decision and it isn't in NATS power to reverse it. The regulated business is kept seperate from the unregulated business by the terms of the NERL Licence and the possibility of infringing European competition law if it isn't. NATS can't use NERL money to keep NSL contracts and even if it could the NERL money is also being continually squeezed by the regulator.
NSL has two choices compete on price or compete on quality of service. The first is a race to the lowest price which it probably can't win. The second may lead to the loss of some contracts.