The decision the MOD faced was either to cancel other defence contracts or delay the carriers in order to adhere to annual spending limits. Therefore value for money has been sacrificed in order to mop up after Labour's mismanagement of the public finances and the expensive Iraq war.
Yet, the delay, and the resultant cost increase, also has consequences too as future equipment contracts will not now proceed.
So Labour are busting the bank now so that it will be impossible to pay for the essential equipment for the future.