If you're only after insurance for a less than a day, a week or few months, then a temporary car insurance policy is worth considering. The idea is this will work out cheaper than taking out an annual policy and cancelling it, where you'd usually pay admin fees.
Using temporary car insurance is often a good option to ensure you are covered while borrowing someone else's car, as you don't need to amend the car owner's existing policy, and, if something were to happen, you wouldn't impact their no-claims discount.
Read more on temporary car insurance.