Not necessarily - it depends on the conditions of your policy whether or not and under what circumstances you may drive another car. If it is not clearly stated on your policy documents then only your insurer will be able to confirm if you have a clause offering Third Party coverage when "driving other cars".
If you can it will be on the certificate. It will say something like..." May drive another car not owned but with the owners permission, not for hire or reward". Fully COMP is irrelevant, it does NOT imply the above clause, it is a common myth that it does.
Not only does it need to be on your policy, and now more often than not it has been removed unless asked for, but the car also needs to be insured by the owner too.