Why did Britain not offer HMS Leviathan to Spain?
Short answerProbably because Britain had little desire to strengthen Franco's Spain with a carrier transfer.
NotesA carrier transfer depends on political alignment as well as the availability of a hull. Britain's poor relationship with Francisco Franco is the likely obstacle. Selling the incomplete Majestic-class Leviathan would have strengthened a regime Britain did not wish to assist. The answer is explicitly a probable political explanation rather than a documented sale negotiation.