Yes, and your certificate doesnt show that! It shows that you can drive a hire car or lease car on your policy, but no other car. It's plain english! Not japanese.
Scenario - you go and get your car serviced, its in for two days, the garage offer you a hire car. You take it. you have two choices; use your insurance which will only cover you 3rd party (so you'll have to pay an excess and the damage to the courtesy car) or you pay a nominal fee (normaly £10) and you go on their insurance (still having to pay the excess if you crash)
For the last time. Your policy absolutely does not cover you for driving any other car 3rd party unless it is a hire car. I've been in this situation before, and yes, I got a ban. Its not worth it!
The policy document is a mass produced document, its whats on the certificate that counts, and your certificate does not say it!.