The car owner of the vehicle you’re driving should be the primary policyholder, even if you’re using their car temporarily. However, whether you’re listed on the policy or not, you may receive the same cover as the main driver. For example, if you cause damage to another vehicle in a car accident, the policy could cover that instance.
Keep in mind that if you’re not a named driver on the policy, the main driver could be charged an unlisted driver excess if you need to make a claim. It’s also worthwhile to check that there are no restrictions on the policy which would impact cover.