There is no commitment period, you can hand in your notice as soon as your ticket comes through the post.
Don't do what one guy did before and post on here (with your full name as a user handle) your intentions to get a licence with NATS and then clear off elsewhere. You never know who reads these forums
I'm not saying your intentions are as such, this thread just reminded me of an event last year.
If you did wish to remain in the UK and were assigned to an Area Radar unit you wouldn't really have much choice anyway!