Insurance wise you normally call your insurance company and tell them you want insuring on the new motor. In my case they sent a cover note to the
dealer and he taxed it from there. All you should have to do is go and collect the car.
Anyone can tax your car as long as they have the right documents (MOT if req'd, insurance docs and V5).
You could sign the paper work for the car and get the new owner slip from the
dealer and then go and get it taxed yourself. You need to fill in a V10 at the nearest Post Office.