I sometimes do grocery shopping through Ocado. The main reason for choosing Ocado rather than Sainsbury's or Tesco is that you can specify the time you want to be delivered in a 1 hour slot and they do respect it. So there's no being stuck at home waiting all day for your groceries.
A nice touch is that there is always a free gift in addition to what you ordered. Sometimes, it can be really good: the delivery I got yesterday included a free bottle of South African wine, Kumala Cabernet Sauvignon Shiraz. Nice!