The pre op diet is to ensure that your liver is in perfect order for the op, rather than weight loss. Straying outside the guidelines could impact on this. However, I'm sure that others may have strayed and still had the op.
My advice is to move on from this, don't beat yourself up, but don't do it again. Your whole future is not worth the gamble of a sausage bap is it? Also, the pre op diet is manageable, we've all done it and gone through a variety of side effects. As far as I am aware, shakes or a headache during this phase will not kill you.
So, get back, more determined and work through it. You WILL be successful.