The Daz Doorstep Challenge is a well-known part of British culture, often to be seen during commercial breaks. It basically involves a B-list celebrity turning up at the doorstep of a stereotypical housewife with children, asking if she'd like to swap her usual washing powder for Daz.
Funnily enough, the woman was always thrilled by the results and the sheets always turned out whiter than white.
I suppose it's possible that they really did turn up randomly at people's houses, but then again I don't know anyone it's ever happened to. It's more likely that they employ actresses.