I know that most non NATS airports require student ATCOs to sign a bonding agreement before they go on ATC courses, they usually last for three years and they are legally enforcable.
As with others, I do know of some people who have walked away and not been pursued, but I also know of 6 people who have done that and been chased to the ends of the earth - in all 6 cases the candidate settled just before court action commenced.
If an employer is prepared to spend many £1000s on training you, you should be prepared to return that commitment by staying for at least the bonding period, or be prepared to repay the proportionate amount.
If you don't think along the same lines, consider that any future employer is going to know that you are an untrustworthy and dishonourable person who will only be employed if they (the employer) is absolutely desperate and they will go to great lengths in ensuring they don't invest any money in enhancing your career.
Aviation is such a small world, that they'll probably get this information by word of mouth before they get any references.