What if I change my mind and decide not to sell my property, after contracts have been exchanged and prior to completion?

You would not be able to trigger ClozeSure.

You will be in breach of contract and almost certainly be liable to return the deposit to the buyer (who may bring a claim against you for the costs, damages and losses they have incurred due to your breach of contract).

Note that you will not be entitled to a refund of the ClozeSure fee.