Arena 33
As had been pointed out it is very difficult to get into NATS as a Trainee Air Traffic Controller. If you did I believe you are bonded to them for five years, however I do not know from which point that starts. Also be aware that you can be posted to any of their units not necessarily the unit of your choice.
Also be aware that if your OJTI's at the unit find out that you are only using them to become a pilot they may get a little bit p*ssed off!