if your mum lacks capacity now, you will not be able to do an LPA anyway.
As regards to medical treatments and health and welfare decisions, no-one can give or withold consent on behalf of another adult person anyway, and if the person lacks capacity to make (for example) healthcare decisions then the doctors can make a decision in the persons best interests. It is common practice to ask relatives what they think, but there is no legal compunction act on what the relatives say
so in summary - yes, save your self the money (although it actually only costs �150 if you download the forms and register it yourself) as it wont be valid anyway if your mum lacks capacity now