I'm not sure that the renting of the property at the going rate, applies here - he's buying the house, it's not being gifted. If your father was to give you his house, then you have to rent it at the going rate, etc and pay any taxes due on the rent, etc.
.... but the house is being bought outright, so I would think you can do what you want with it and that would include let him live there rent-free. The main issue, would be what happens to the proceeds of the sale, ie the £100k.