Firstly selling you the property at less than its market price means she is making a Potentially Exempt Transfer (for inheritance tax purposes) for the difference between what it is worth and what you pay for it. Fine if she lives another 7 years, less than that and the sum goes into her inheritance tax liability.
Secondly if she is doing this to reduce her net assets that could be used to pay her a care home, local authorities are wise to the ploy.
Thirdly, regarding the house/land itself, you do have to transfer the land title to you at the Land Registry - otherwise she still owns it. A solicitor or licensed conveyancer is the most common means to doing this.