Most of the international branch operations I've come across are separate from the home-country systems...AFAIK none of these banks have significant retail operations in Spain (not sure about Santander Serfin).
And you should also check what "surcharge free" means. A friend of mine has accounts with HSBC in the UK and Greece. There is no surcharge for withdrawals, but money is converted at the retail rate so he ends up paying over 5% more than he would at a non-allied bank that would charge a surcharge but use the interbank rate.