The UN has no troops. Any binding Security Council resolutions preventing Britains reconquest would have been negated by Britains veto powers, never mind the possible French and US veto powers.

The UN DID pass resolution 502. but that was fairly tame and was aimed at stopping the Argentinian aggression and suggesting everyone sit around and work things out. Meanwhile Britain invoked Article 51 of the UN Charter regarding self defence, and freed the Falklands.

Its very hard to prevent the countries with mandatory veto Powers at the UN from doing stuff. Russia recently vetoed a vote condemning their acquisition of Crimea for example.

Source: The UN http://www.un.org/en/ga/search/view_doc.asp?symbol=S/RES/502(1982)