Why should you need cover? You are thinking about this from the wrong perspective. It is down to the prospective purchaser to have insurance in place, not you. If they want to drive your car, then they need to insure it, it is as simple as that. They can arrange for cover through companies like Dayinsure or any other insurer that provides temporary car insurance.

If someone is looking at spending £20k, they can afford the small cost of insuring it for half an hour. No insurance, no test drive, it's as simple as that. Alternatively, you drive the car, and they can experience the test drive from the passenger seat