hand the keys to the house to the bank, and liquidate the loan. If the house value is greater than the value of the outstanding loan, then you're out of the agreement.
HOWEVER, the current projection of a saving of Â£1500 is based on interest rates at their current level. Depending on the rate it may fall to, you could potentially save sufficiently to warrant the change, but only if you change now as the Â£2k redemption fee is unlikely to change significantly. That has quite an element of risk (although the reality is that rates are likely to fall to virtually zero over the next 6 months), and the fact that you fixed at 6.9 suggests you are fairly risk averse.