There is a little store just outside the duty free area where you can find Walkers and chocolates cheaper, but still airport prices. Last time I was there I bought Walkers in the duty free and in the other store to split my losses.
Also I would give a try to Fortum & Mason, it is really expensive, but it is worth it. Love the lemon curd from there.
That's a good way to buy products to lower the average costs. I never thought of buying lemon curd from Fortnum & Mason, even though I know their hampers are very popular. Maybe I'll poke and see what's inside their hampers next time, they must include all the most popular items