... but you weren't told it wasn't a hire car. My opinion is that, unless there is a serious fault or the mileage reading isn't correct, you should keep the car. It's worth contacting the dealer to see if they feel different. Some dealers do offer a no-quibble return policy, others don't. Your best option is to appeal to their decency.
Under certain circumstances, the law allows you to change your mind within 7 days of taking delivery with no penalty.