This article by the RAC caught my eye. It looks at circumstances where you may find yourself uninsured and facing 6 penalty points and a minimum £300 fine.
These are the main situations where people get caught out:
1. Test driving a car
2. Using a car for a business trip
3. Driving friends and family car
4. Borrowing a vehicle for a house move - loading the car the wrong way
5. Sharing driving on a long journey - whose insurance policy is covering you?
6. Switching policies - many people get the switch dates wrong!
7. Insurance for a new car (new used car!)
Take a look at the RAC website to see what they say.