The other replies are correct. The company will send you where you're needed, it's safe to assume you'll have no choice in that. If you absolutely can't accept a posting to, say, Aberdeen airport, or the Prestwick centre, then you have indeed answered your own question! My first airport posting was several hundred miles (and in a different country) from where I'd wanted to be, although with hindsight it turned out to have been the best experience I could have wished for (plus I didn't have a young family at the time).
Despite some of the grumbles you'll read on here, NATS really is a very good employer, and ATC is one of the best jobs in the world. Good luck with whatever decision you make.