AFAIK so long as your drivers license covers you to tow a trailer then you are OK. I think most UK licenses do as a matter of course. But if it's a Spanish license then I don't know.
Also, Leojohn is right about the actual vehicle documents, the tow bar has to be shown on the ficha technical. But I think that its common practice for people just to remove the tow ball at ITV time to avoid any issues.
Hope that helps a little.