Why would you be able to claim tax relief for breaching the time element of your contract?
The bond is a security to protect the employer for losses they might incur as a result of your electing to terminate your employment within a specified period. You are free to take this course of action, but would then be liable for the terms of the contract you entered into.
Why should you then be entitled to claim relief on those monies. Anybody breaking a mortgage contract or another financial agreement would not be entitled to relief on any penalities.
This isn't money that you have expended as a consequence of your employment, it is money that you have expended as result of not fulfilling your contractual time obligations. That contract or bond was presumably entered into in order to put you in a position to undertake that employment, and in the UK that is not allowable for the purpose of tax relief.