Hi Howie
Without reading your Poder I do not know why you have been advised to make another. I would have thought that if you have one which authorises your son to act for you, which includes in a capacity which covers selling the property , I suspect your lawyer may just be after getting another fee from you, and that you do not need a new Poder. The ones my sons have are over 20 years old and they are still `good`.
Thus, I suggest you confirm why you NEED to make another.
Re FCO. If you really do need to make a new Poder, I understand there are `Spanish notaries´ in UK. When I was in the Philippines I used a notary who had an office near the Spanish Consulate. Maybe Google, or speak to Spanish consulate in UK for advice.
Joint Poder.
Again without seeing the document, I suspect that is a power given by each you and your wife. Thus say if you wife was signing for the sale the person you gave the power to could act for you and sign. So in effect the Power as I say is given by each you, and / or, your wife and thus they are individual Poders albeit on the same document.