Asda are selling Huggies Super Dry big packs for as little as £9 each, and I've been able to find them for this price at Tesco, and slightly cheaper at Amazon. However, you can get 2 packs for £15 in the multi-buy offer, and I haven't been able to find this cheaper anywhere. It works out to (for say the Huggies Super Dry 76 big pack size 4) just 10p per nappy.
I couldn't find an option to collect in-store. Home delivery charges start at £3, depending on what times you opt for. Keep in mind that there's a minimum spend for home delivery of £25. Also, don't book a slot too far in advance, as you're charged whatever the price is on the day, not at the time you place your order.
If you shop at Asda, it might be worth going into the store to see what they've got on the shelves. As for buying four boxes, well I remember what it was like when son was still in nappies, and it doesn't take long to get through that many!
Thanks to speaknoevil at HUKD!