That's a popular misconception, Goldencup. A lot of policies have changed in the last few years without any big headlines.
Check your own policy wording. You might be surprised.
A lot of the big companies are taking the third party in other car out of the standard conditions and making it an option.
[Don't assume you're allowed to drive other people's cars
Many people assume that if they have comprehensive car insurance then they are automatically covered to drive other people's cars. This is not always the case. Some insurers no longer offer this cover and those that do don't offer it to everyone. The only way to know is to check your insurance certificate, which will state clearly if you have it.
One more thing to beware of is that the 'driving other cars' extension gives you just the basic insurance cover allowed by law. This is called Road Traffic Act cover, which is even less than third-party only insurance. The nub of this is, if you have an expensive accident whilst driving someone else's car, you may end up being liable for part of the other party's claim!]
http://www.fool.co.uk/insurance/information/car-insurance.aspx