Don't destroy your expired UK Passport - certainly not if you have property in Spain. Because, when you bought it or obtained your Spanish Tax number for your NIE, the Spanish tax system will have logged the number of your passport. If that passport has expired now, you have a different number on your new one. That blows their Spanish minds - because in Spain your passport number is for life. So don't throw away your old passport.
If you have already done so, and you live in UK, don't panic, the Passport office can help you out . Read this link by my pal Alex Radford, lawyer in Spain. https://www.mylawyerinspain.com/blog/changes-to-requirements-when-selling-property-in-spain/#comment-132