Coldaba you are right that if you keep the boat British registered then the rules are grey to say the least regarding the licensing of the boat . As far as you as skipper are concerned though they expect you to have a Spanish qualification if the boat is Spanish registered BUT expect you to have a UK qualification if it is UK flagged. The fact that the UK authorities  do not insist on anything doesn't mean you will get away with having nothing out here. They are probably looking for an International Certificate of Competence (ICC) as an equivalent to a local qualification.