Theoratically she doesn't if she is travelling together with you and you are armed with your marriage certificate but.........practically you can well be stopped either at the airport on arrival and even at check-in before departure. You can show your marriage certificate and a printed copy of the EU directive on free movement of family members plus be ready to be stopped for hours and argue with officials. In simple words, you will have to convince the check-in staff in UK and/or the immigration officials in Spain that she doesn't need a visa when they think she does because they are not aware of her right to free travel.

If you are ready for that, then go ahead otherwise get a schengen visa. I would go for the latter if I were you !

Good luck