Recently, our local Asda (and others in the midlands) have been stocking proper Kraft Dinner, in the blue box for £1. Caution, use more than the 125mL of milk the recipe calls for, or else you'll have pasta with cheesy cement.
The other stuff they stock is called "Cheesey Pasta" (yes, the spelling is incorrect). It's made by Kraft, but it is more akin to the cheaper supermarket-brand Mac & Cheese you'd find in the states.