Not being funny here, but why? You’ve got full residency rights in Spain, and a UK passport. If you get a Spanish passport then you lose the UK one. The only advantage I can see is that it’s easier to settle in another EU county (or ex Spanish colony) if you decide you want to.
A friend got an Irish passport the other day. What a prick. Very jealous.
3 more years and I can get a Spanish one.