This is the answer to this question from the Direct Line website.
If you have �Driving other Cars� extension then you will able to drive a vehicle �not owned, hired or leased to you�, this extension only applies to the POLICYHOLDER, but there are age and occupation restrictions. Cover is also restricted to the minimum legal requirement of �TPO� (i.e. if you have a comprehensive policy the cover for driving other cars is restricted to TPO).
If you search this site you will find lots of comments and opinions on this subject.
This facility is being withdrawn by many insurers because it is often abused.
For an authorative answer contact your insurers and see if they insist that the other car is insured by its owner. On the Direct Line website it makes no mention of this.
You have to be aware however that if you do use another persons car you will have only basic third party cover and any damage caused by you to the car would not be covered.