Depends whether you work on aerodrome or area, if you get area which most people go into, you will be based at Swanwick or Prestwick, if you get aerodrome, you could be based at any of the 15 (I think) aerodromes that NATS controls.
Either way, you have to sign a mobility contract at your stage 1 which means you agree to move wherever the business needs you, so you need to be flexible.