Just an impartial observation:- I could very easily name a dozen or more ATCOs who voluntarily left IAL/SERCO to work for NATS. I would find it very difficult to name more than two or three ATCOs who voluntarily moved in the other direction, and two of those subsequently returned to NATS.
I'm not trying to start an anti-SERCO debate, but NATS employment T&Cs, despite all its apparent faults, are still hard to beat in the UK.