There are two Marks & Spencers within St. Pancras International Station. This one sells clothes, food & drink, the other only sells food and drink, located in The Arcade
Decent sized shop in the train station. You can get almost anything you'd need from here. Good for stocking up on snacks before long train journeys. Friendly service.
5.30 single can of lager drinkers - club together and buy a 4 pack. 4x shorter queue + less futive looks over the shoulder, worried about missing the train and getting in trouble with the wife.