You can check if you are insured to drive someone else's car by reading your Certificate of Motor Insurance.
You are insured to driver other cars if your Certificate says:
"The policy holder may also drive a vehicle with the permission of the owner of a motor car not owned by the policy holder and not hired to him/her under a hire purchase agreement."
(You should see this under the section 'Insured persons')
If your Certificate does not contain this phrase, you are not insured to drive other cars.
Please note: If you are insured to drive other cars, your cover is only Third-Party (Damage to another person's property & injury to another person or any passenger(s) you are carrying).