I lived in London for 5 years and used grocery delivery services a lot. Now I live in Seattle, and don't use them anywhere near as frequently.

There are many difference between grocery shopping in the US and UK cities, but by far the biggest is that in London, a lot of people live without a car. This means walking or taking the bus to the supermarket. (Eg: in San Francisco, 28% of households have no car [1]; in London that number is 60%. [2])

Home delivery allows Londoners to buy the type of and volume of products you want. (I'd rarely buy a 6-pack of 2-liter orange juice cartons alongside the rest of the groceries when I had to walk 30 minutes back from the store.)

Without a car, you also have to make more frequent trips to the store, increasing the overhead of shopping. Being able to drive to the store for a 'big shop' every week or so is much more efficient.

[1] http://en.wikipedia.org/wiki/List_of_U.S._cities_with_most_h... [2] http://cyclelondoncity.blogspot.com/2012/12/census-data-car-...