The penalty for an overpayment (above any allowance like Avant's 10%) is calculated using essentially the same formula as is used for calculating the break fee when switching lenders. But instead of the outstanding mortgage balance, they use the amount you want to overpay.
This means that the penalty will depend on:
- how much you want to overpay
- how long is left on your-fixed rate period
- how much interbank interest rates have moved since you fixed
So it is pretty much impossible to predict, except to say that if interbank interest rates rise after you fix, the penalty will be zero.