If what OLNEY 1 B says is correct maybe there is some truth in TBI's assertion that NATS always go for the same gold-plated equipment. I hear the radios at LACC sound much the same.
More seriously, the near monopoly position that NATS has at the UK's major airports is based more on history than on the service provided by the current contractor. Things have grown up with NATS doing ATC - and to be fair they probably do a pretty good job - but the business risks of changing to another contractor are simply too great for the likes of BAA plc and MA plc to seriously consider.
Although NATS may provide a good service, other companies could provide an equally high quality service - and many airports directly employ their own staff very sucessfully, as Luton did not so long ago. The staffing and training issues are not insurmountable but would need careful management. But can you really see the Board members taking on the risk of it not working out well?