If you are buying a car or van you will need some form of driveaway insurance in order to bring it home.
If the vehicle is new or you are buying from a large dealership they may have an arrangement with an insurance company under which you get free insurance for a short period of time, usually with the proviso that you get a quote for a standard yearly policy, which you are not obliged to accept. Nothing comes for nothing of course, so the cost of this insurance is usually factored into the price of the vehicle, something that you may wish to use as a bargaining point for a price reduction if you don't need it! If you are buying privately however, or from a small dealer, you are left with the options of either taking the risk of driving it home without cover (very dodgy, particularly near to auction houses where the police look out for this type of offence), leaving the car where it is until you have arranged a yearly policy on it, or buying a short term insurance policy to cover you for the time it will take you to get it home and arrange for full insurance.
A 'free' driveaway policy from your dealer can be a very attractive proposition particularly since some of them last for seven days or even more. It is important however to make sure that the level of cover is sufficient for your needs; it could prove a poor bargain if you damage an expensive car only to find that the cover is insufficient to pay for repairs, or that you have to pay a huge excess. Buying a short term car insurance policy can mean more preparation work; if you know the vehicle's registration details you can arrange this before you collect the vehicle, but if you have a choice of several cars to buy you would need to have access to the Internet in order to buy the cover. However most auction houses and major dealers provide free Wi-Fi and most of us now carry smartphones so this should not prove to be too much of a problem. Comprehensive cover can usually be arranged very simply and the whole process from obtaining a quote and buying a policy, which can take effect immediately if you wish, only takes a few minutes.
Incidentally, please don't fall into the trap of thinking that an 'any car not belonging to you' clause in an existing car insurance policy will cover you to drive a car home after you've bought it. It won't - and police forces and courts hear this excuse regularly, and it cuts no ice I'm afraid!