You don't get the "Third Party Extension" anyway in the majority of cases anyway, especially as a young driver as in nearly all cases you have to be 25 or older and be fully comp to do so.

People also misunderstand this massively, it's basically designed for emergencys or one offs, there are many restrictions imposed, I.E the car cannot be owned by yourself nor can it be a hire car etc. Most companies are stopping this full stop as alot of people were abusing it, I.E insurance fraud! Also it only covers you for the bare minimum for you to be legal on the road, so if the car gets stolen for example you would not be covered!

Martin