Thanks for your enquiry.
As the Deeds are in joint names, your wife is as much a legal owner of the Property as you are, even though she left 12 years ago.
Likewise, if you were to sell, she would need to sign the Contract of Sale.
I am guessing that you never got divorced, and by beginning such proceedings may be the only way you can resolve this issue, if she is unwilling just to transfer the property over to you. The house and any other financial issues can be dealt with within the divorce proceedings, by the parties agreeing to who gets what.
Not sure if you have ontemplated a divorce, but if you do not, then the only thing you an do is contact her and see if she is willing to transfer the house into your sole name. If she is, you would then need to instruct a local Conveyancing Solicitor to prepare the Land Registry Transfer document which would need to be signed by the two of you, and which is then registered at the Land Registry.
I hope this answers your question, and if so, I would be grateful if you could leave positive feedback.