To Cool Breeze about Champions League. Actually I would not have known whether to put an apostrophe or not in this case (I'm not a football enthusiast). In general you're right, that plural nouns are not often used adjectivally. the exception is when a noun is normally used in the plural. As we can talk about a single tooth we say toothbrush, not teethbrush, even though we (usually) have more than one tooth. On the other hand, "clothes" is
always plural, so we say clothes-brush.
A digression about "soccer", which you say is AmE. Historically this is a BrE term and was coined to distinguish football according to AsSOCiation rules from Rugby football (sometimes called "rugger"). However, if you mention football in Britain, people will assume you arer talking about soccer, so the term has become more useful for the Americans, to distinguish from American football.
Cheers
Lewis