With the open European borders it could be anyone, DFS, AENA etc. Take your pick.

BD

DFS and AENA are far too busy putting their own houses in order to even contemplate looking at NSL territory. NATS however has the potential to look at AENA and DSL contracts along with many others in Europe, Asia, Middle East etc. The biggest potential threat comes from SERCO, however they do not have the staff to service a large UK contract and are unlikely to get sufficient support from current NSL staff to make it work.

Market forces.Experienced Approach Radar controllers are in increasingly short supply.About time Nats/Prospect thought about this.If another ATC supplier came over the horizon,then some might be tempted.NSL at the lower band units don't have a good record at fair treatment compared with NERL.Screw up the pension and a lot might walk even quicker.
Just remember market forces doesn't always mean a reduction in T&C.Look at the kicking the North Sea Divers gave that industry.

Doesn't matter too much (initially) who might take over a contract as TUPE would apply. That means T&C's are protected until such time as they are renegotiated by Prospect and the new management. But beware of Prospect - they are just the negotiating arm of management.

AFAIK TUPE protection excludes pensions. And what is viewed as the most expensive part of the current remuneration package?