Insurance while buying a car

I've just returned to the UK after several years overseas. I'm looking to buy a car (second-hand) but have no insurance. I'm getting about in a hire-car.

I know of one day car insurance, but I'm looking to try out several cars before I buy one, and a one day insurance policy - as far as I can see - covers only one specific car.

Any suggestions on what to do to ensure I'm covered when taking out numerous cars for test drives?