I sit in the car on my mobile with my credit card and arrange the insurance.
they usually give you the policy number.

It's best to already have already obtained quotes for the type of car you want to buy so that your preferred insurance company is already chosen when the time comes to call.

If you are confident you will be coming away with the car, you can also arrange it before you set off from home and cancel it within 14 days if you don't.

UK insurance rules state that consumers have a 14-day “cooling off” period when they purchase most kinds of cover. This means that, if you want to end a policy within two weeks of receiving your documentation, you should receive a more or less full refund of your premiums. But if you cancel within the cooling-off period, you may have to pay an administration fee (typically less than a full cancellation charge) as well as the cost of however many days you were covered for.

So if your policy has not yet come into effect when you cancel (because you didn't buy the car), you should only have to pay the administration charge. You won’t be able to cancel in this way if you have made any claims within the cooling-off period.

Edited by Hamsafar on 15/09/2013 at 12:44

Ask a friend/family member who already drives to get insured on it for the day to get it back to yours.
I think when I went to get my first car my dad got insurance out on it for the day for something like £20.