I use Sainsbury's online and have it delivered free as part of my grocery shopping - with top ups at the Coop locally if needed. I know that other posters use online pet stores though so would be interested in comparative prices and ease of delivery.
I use pet planet for litter because we use chick crumb. They've got free delivery on orders over 39 quid. Not sure how good they are for food though. I've noticed that Amazon has subscribe and save on some pet food so if you book a delivery for once a month or whatever it's cheaper than a one off. Their whisked 48 packs aren't as cheap as Asda but the last time we ordered cat food from them and chucked in a few things for us they brought us everything but the cat food.
Another recommendation for zooplus here. We have 3 cats and one requires expensive food due to his digestive issues which means that they all have to eat that food. We found zooplus to be the cheapest place for their food and cat litter. We bulk buy 40l of litter and 2 months worth of food at a time. It also means I don't have to carry it back from Tesco anymore.
Zooplus recommendation again, I get Bozita & bulk buy tons - free delivery & it's on cash back websites. Tesco's own antibacterial cat litter is the best I've found, it's just like Catsan but much cheaper!
Zooplus or Amazon for food (whichever is cheaper at the time, although I prefer not to use Amazon now). They'll only eat ridiculous Italian pouches atm because they are divas, so wherever is cheapest gets my money!