Well I dunno really, I'm just saying no one seemed to need any documents with me last time (Ford). Could be because I took their insurance, but IIRC the dealer still didn't have anything and had to get the cover note faxed over as I was picking the car up. Perhaps they used a different document that they then keep?
Its a bit silly anyway since there is nothing to stop anyone taxing a car then immediately cancelling the insurance. The government has no way to stop you taking the 7 day cover and then letting it lapse. Why would they even care anyway? They take your money for the VED, then let you get fined for braking the law later. Win win for them.