In a former life I used to work for an Insurance Brokers and hubby is an Insurance Underwriter.
In most cases, people with fully comprehensive insurance are covered to drive other vechicles on a third party cover. This basically covers you if you have an accident, well it covers the person you hit and there car. The car you are driving is not covered at all. Therefore I would only reccommend driving on this cover in emergencies. If you want a policy that covers you on all vechicles, fully comp then you will pay for it or you need a business car policy. (this is what mobile tech's should have, class 1 business use.)
Car Insurance for the young is expensive, It has always been that way and always will. Most accidents are caused by new and young drivers. When you turn 25, you will see a big difference and your NCD will count.