There is no tax on gifts, per se.
However, if the parents were to give their daughters a substantial gift (such as the house), and both parents were to die within 7 years, the gift would be taken into account when assessing the value of the parents' estate for inheritance tax purposes.
If the daughters purchase the house, from their parents, for the full market price, no gift has been given and there could therefore be no liability for inheritance tax in respect of the value of the house.
However, if the parents sold a �300,000 house, to their daughters, for �100,000, the tax man would regard this in the same way as if the daughters had paid the full market price and the parents had given �200,000 back. i.e. if both parents were to die within 7 years, a gift of �200,000 would be taken into account for inheritance tax purposes.
Even so, as long as the family don't fall out (because the daughters, as owners of the house, could seek to evict their parents), the proposal could make sound financial sense. If the second parent to die still owned the house at the time of his/her death, the full value of the house would count when calculating inheritance tax. However, if the house had been transferred (even with an element of 'gift' involved) to the daughters more than 7 years earlier, there would be no IHT to pay in respect of the value of the house.
Chris