Ocado is definitely the best. They deliver mostly Waitrose stuff with some of their own range.
Website very quick and easy once you get the hang of it. There is an 'add all' button which puts your usual groceries in your trolley, and then you just get up the 'pictures' version of the trolley and go down it, adding anything else you might want that week, or deleting anything you don't need.
Delivery slots are one hour (usually two with the other lots). Charges vary with time of day/ day of week. Annual contract, where they take the fee monthly is the cheapest way to do it.
Deliveries over £40 (very easily reached!). As often as you like when you have the Ocado delivery pass.
HTH