Yes you do need to be very careful when executing powers of attorney as there have indeed been cases of Spanish solicitors, estate agents and the likes mis-behaving. I always recommend solicitors with dual practising certificates (ie UK/IRL and Spanish qualified).
Your Spanish solicitor should easily be able to draft a dual column (ie Spanish & English) power of attorney for you to execute before a notary in the UK. If you are intending to appoint them there should be no additional fee or at most EUR100.
You can then go before a Uk notary and they will witness the execution. Fees do vary slightly but should be somewhere between GBP40 to GBP60 per notaries signature. You will also probably require a notarised copy of your passport. You can use this link to find a UK notary
The documents will also require legalisation or as it is also known apostilling to be valid for use in Spain. Most notaries offer this service but if not you can post the documents off to the Foreign and Commonwealth Office in Milton Keynes with fee of GBP27 per document plus GBP6 postage. Check out the following site
Of course you can get UK notaries to do all the drafting – there are many that specialise in Spanish transactions – but for that service they will charge you a lot more.