How about getting trainees to sign a contract, which states something along the lines of
"Upon successful validation at a unit you shall take up a debt, hereby known as a training loan, to cover training costs. Repayment shall be waived unless you leave NATS (through disciplinary dismissal or VR) within a period of 5 years (for example) of your validation date.
Validation date is the date at which you satisfy the unit MUR.
The amount of the training loan shall be £80k (or any suitable figure).
The following costs shall be incurred
Leave within
1 year 100% to be repaid
1-2 years 80% to be repaid
2-3 years 60% to be repaid
3-4 years 40% to be repaid
4-5 years 20% to be repaid."
after 5 or more years - debt cancelled"
Not a bond per se, but will discourage cutting and running to sunnier climes until we get some return of service. If an ATCO was later found to be useless and hade their licence removed, then that would be a get out of jail free card as it's not disciplinary.
Only successful ATCOs who leave NATS of their own volition, or by doing something to upset the applecart would have to pay.