Buy over the internet and you're usually covered by the distance-selling regulations, which are part of the Consumer Credit Act. This allows a seven-day cooling-off period after delivery, and a full refund if you're not happy.
As a result, you only reserve the car online, and are under no obligation to accept it when you see it in the metal.
If you choose to walk away, the company will refund your deposit, and if you go ahead you're automatically covered under the distance-selling regulations. So, buy the wrong car or discover a problem and you're guaranteed a full refund by law.
However, very few buyers actually hand back a car: internet sellers reckon less than 2% of vehicles are returned.