This is new to me....
http://go.channeladvisor.com/rs/channeladvisor/images/uk-op-tesco-fact-sheet.pdf

Looks like it is by invite only, but as they have 50 retailers it will one day be of interest I am sure.

I think it launched nearly a year ago, but it has been slow to grow due to the invite only nature. They were talking to a lot of retailers back then. Not sure why they haven't pushed it more.