Even though you are the sole earner because the mortgage is in joint names the Mortgage Company will not agree to changing it to your wife's sole name. Once the mortgage has been paid off you can change the ownership of the house if you wish. Phone the Land Registry Office local to your address and they will help you. You should be able to complete it yourself with a small (less than �100) fee payable.
Just to be clear, is it not possible to transfer the deeds of the house to her while I'm still paying the mortgage on it (or while we just have a joint mortgage)?