I don't know how long this has been going on for as I have just received the email but you can now download video games from amazon.co.uk as well as purchase Xbox LIVE Memberships and Points from their digital points and cards store.
From what I can see you can download:
PC Games.
MAC Games.
Free to Play Games.
Xbox LIVE Membership and Microsoft Points codes.
And they have a "Digital Deals" section where, by the looks of things, you can download games and game DLC at good prices.