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You apply for a multiple-entry Schengen (short-stay) visa exactly in the same way than for a single entry Schengen visa, filling in the form accordingly. Your best bet is continuing to apply for a visa for each holiday. Once you have been there several time and can show that you used your visa appropriately, you will have a better chance to get a multiple entry visa. Being a UK resident should be in your favour too. Finally, you could write a letter to highlight all this and make a case for that visa and include it in your application.
But which visa is granted is mostly up to the Spanish consulate. None of this provide any guarantee.