Optimum Nutrition Protein Shake

£1.99 (per 330ml bottle), optimumnutrition.com

This is a great-tasting, ready-to-drink milkshake, with 25g of muscle-repairing protein per bottle. With just 161 calories, it also beats a lot of its rivals in terms of kcal count, and comes in four flavours: Chocolate, Strawberry, Vanilla, and White Chocolate. Each is sweetened with the artificial sweetener sucralose.