So this was a little project inspired by this comment on a reddit thread where someone joked about figuring out how many kilos of greggs they could but with their income...
Rather surprisingly, the product with the most grams per pound to it's name is the Sparkling Mango and Pinapple drink (link), for which you'll get 277g per pound spent!
The most inefficient weight for price option, at a measly 21g per pound, is the Spicy Mexican Chicken Pizza (link for pepperoni pizza, the mexican one doesn't have a listing it seems)