Five shots, preferably but not necessarily, of the cheapest hard-liquor in the bar. The shots are lined up in a row and consumed, one after the other, in rapid succession. It is often used by people who, having arrived at the bar or club late in the evening, need to catch up in order to have a good time.
Mike: "Shit, I'm late and all you fuckers started drinking without me."
Tom: "No problem bro, just do The London Classic and you'll be good to go."
Mike: "Fuck, I knew you were going to suggest that." *Facepalm