If one spouse is Spanish, the marriage can be registered with the Spanish Civil Registry, either through the Spanish consulate covering the place of marriage or through the Registro Civil in Spain. The London consulate currently asks for a British marriage certificate issued within the last 6 months.
Here's the useful twist: for marriages celebrated within its consular district, the Consulate General of Spain in London says documents in English, French or Portuguese don't need translating. If you register there, you may not need us at all, and we'd rather tell you that than sell you a translation nobody reads.
Registering through an office in Spain is a different matter, and each Registro Civil can ask for a traducción jurada of a foreign certificate. Once the marriage is registered, the Spanish entry is what Spanish authorities will rely on in future, which saves translating the British certificate every few years.