Training bonds are enforceable under UK law and if you renege, your old employer is likely to write to your new employer informing them that they are party to your defaulting on your agreement and asking them to assist you to repay any money outstanding. In my experience this is usually effective and cheaper than going to court.
Well drafted bonding agreements have a clause which specifically states that the bond is not repayable in the event of redundancy or death.
RHRP