It is a myth that every Fully comprehensive cover will allow you to drive a relatives vehicle Third party cover. It is down to individual insurers policies.
You Mums insurance wouldn't cover you unless you are named on it. If they allowed cover for you, then your mum could insure her rusty little ford for £200 and then you could drive your £100,000 lexus. EVERY insurer would go bust if they offered that kind of cover.
For insurance advice mate, you need to read this
http://www.squidoo.com/tips-4-car-insurance