I guess it comes down to the previously mentioned precedence and case law. At the moment there is none in the UK, so bonds are neither legal nor illegal until ruled on one way or the other.
Advice I received is as long as the bond is "reasonable" (financially-for the training being done), decreasing over time, and for a fixed period (upto 5 years) you should pay the outstanding sum back if you leave early.
Bring on that brave person prepared to make case law!!!!!