Whatever the reason, whether buying a second hand car, borrowing your friend’s vehicle or just testing a car you may need a short term car insurance.
There are not many companies in the UK who would insure you just for a few days, perhaps is too much hassle for them and most of the insurers prefer to concentrate on the "annual" customer and they in my opinion are missing on a huge market.
Be careful though and read their terms and conditions as your particular case for example may not be covered. For example short term car insurance would not cover your journey if it does not terminate in the UK, so if you are exporting a vehicle this perhaps is not the appropriate insurance for you.