We have used a spanish firm of lawyers which will supply you with a document in Spanish translated into English (this is the double column Maria is referring to) which can then be signed at a notary in the UK, and apostilled at the Foreign & Commonwealth Office which makes it valid in Spain under the Hague Convention.
There are several different costs so it does add up - having the document drawn up, notary fee, and apostille fee. The notary may offer to arrange the apostille stamp and will charge a fee on top of the Foreign & Commonwealth Office fee, but there is good information on the Foreign & Commonwealth Office website if you want to do it yourself.
Happy to supply details of the lawyer who helped with drawing up the POA.
It would be cheaper to do it at the Spanish Embassy or Consulate if you can get an appointment. The information is on their website.
Even cheaper to do it in Spain, where the notary fees are cheaper, but you would have to have someone with you who can translate into English for you.
Happy to provide details of lawyer etc.if you send me a p.m.
You should also think carefully & take advice about what you want the POA to cover as it could be very specific to your property issues or more general which could be useful for other matters.
I hope this is useful.