Without your names on the deeds you won't get a mortgage and will be limited to an unsecured loan at higher interest rates.
You will need planning permission to separate your parents' house into two dwellings. If your parents are elderly and will require care, you may have a decent chance of getting this, but it's not guaranteed, and you will likely end up with planning conditions that state the property must be returned to a single dwelling once you or your parents move out. So ultimately this might be an expensive solution.