I think there is a false impression that you're "bound" for six years. You're really bound for four years... you can choose not to take the grant/book allowance so you have nothing to pay back if you decide to leave after graduating. Alternatively, you can take the grant/book allowance and return those amounts if you do decide to leave after graduating. You can also opt to, like some of us, take on three subjects instead of two during your part time study in order to finish your degree earlier. You'll finish your degree in 3.5 years and therefore be bound by six months less as it is pro-rata.
Don't forget, if you do leave you'll perhaps be leaving for a better opportunity somewhere else and if the competing firm really wants you then they may even pay your outstanding book allowance/grant.
It's not recommended to leave straight after graduating if you intend to do your CA... you may as well stick it out for your CA as you'll have a wealth of information and a support network.