<<but is it possible to once you have been validated and become an ATCO to then apply to a Non-Nats airport like Leeds, Liverpool, Blackpool etc......>>
In very basic terms, if you reach that stage you will hold an Air Traffic Control Licence. However, in order to use it you must have the appropriate ratings. To work at an airfield you would need Tower and maybe Approach Radar qualifications which you may not get if NATS decide you are going to a Centre.
<<Is the salary really worth the job?>>
Are you kidding? If you are employed by NATS you will have a damned good job with excellent pay (50-80K depending on where you are posted), plenty of time off and good working conditions. Some of the youngsters out there may not agree because they don't know on which side their bread is buttered!