A Nats spokesman rejected the complaint, insisting airports had a choice of suppliers.
This is indeed true; there is nothing stopping TBI finding an alternative service provider if they feel uncomfortable with the costs imposed by NATS.
The fact that NATS provides the service at 14 UK airports is probably indicative of the level of service provided. However, in no way do these 14 constitute a monopoly, or even a near-monopoly, on aerodrome services. NATS does not provide aerodrome or approach services at East Midlands, Liverpool or Leeds, to name but three airports with similar traffic levels to the three owned by TBI.
The whole airline industry has suffered financially following recent events; TBI, as an American company, have probably suffered more than most and are attempting to cut costs for next year's budget.
If TBI have serious concerns with the cost or level of service provided by NATS, then they should indeed consider jumping ship and getting another supplier. However, despite their grumblings in the press, they probably realise that if they pay peanuts, they could well be lumbered with monkeys.