Dumpman, just to emphasis what Team GB said.
The ONLY legal ID Brits have in Spain is their ORIGINAL passport. Not a copy, unless that copy is authorised by the British Consul or by Passport Office.
Having said that, on a day to day basis many people will accept other documents, but that is ether because they are being accommodating, or more likely because they do not know the law. However if you go to sign a document before a notary, to a court, the police etc. unless you take your passport you may find that anything is not accepted.