I think the garage actually owns the car for a minimum period (30 days or 90 days, cant remember which) where it is registered on their fleet and then the car becomes yours. This is for tax benefits I think, so technically you dont own the car when you pick it up and this could apparently affect your insurance.
It is a really grey area at the mo and the rules seem to be bent alot so alot of people steer clear because it isnt clear what the deal is. A lot of people also arent told that the car is pre-registerd and find out after they have taken delivery of the car, this is quite bad on the seller for not informing people of what is actually happening and why they are getting a cheap car!