You could lose a lot of money that way, as well as adversely affect your credit rating. The mortgage company will have to get a repossession through the court - there is no such thing as 'cancelling a mortgage'.
So there will be court fees and other admin charges. The mortgge company will sell as quickly as possible, and if they don't raise enough money to cover your mortgage and all the charges, you will have to pay the difference.
If they sell it for more, you will be the excess profit.
Why not sell the house in the normal manner?