A booze-up is a party, or a whole evening, whose real point is drinking a lot of alcohol. It is informal British talk, a bit old-fashioned and laddish; you hear it from people joking about a big night, not in careful or polite speech. It does not have to be a fancy party — a night in the pub that turns into round after round can be called a booze-up too. Americans are more likely to say binge, bender, or just a night of heavy drinking. Though actually, people sometimes use it with a shrug, as if the mess is part of the joke: nobody is pretending it will be a quiet evening.