The posters above are correct, there is no form of bonding for a NATS ATCO. However, the vast majority of NATS trainees are for Area and there is no other employer in the UK for this discipline. You could try and take your Area ticket overseas but I would imagine that an unvalidated rating would be of no use in getting a job. Similarly, I doubt that an unvalidated Tower or Approach ticket would carry much weight overseas. The Middle East has a big ex-pat workforce, one of their number could comment on this and suggest how much valid experiance their units would look for. I would imagine that it wouldn't be much less than 5 years.
A Tower and/or Approach ticket will probably be useful in getting into a non-state airfield here in the UK over the next few years, as the supply of ready made aerodrome trainees has dried up. Perhaps NATS is relying on superior T & Cs to keep its new ATCOs?