What will they allow - you do go fully comp, or have the driving other cars extention?
In regards to fully comp - most (who will quote for younger drivers anyway) will allow you to quote for a comprehensive policy.
In regards to DOC (driving other cars), this varies insurer to insurer.
DOC is only to be used in an emergancy - police are clamping down on the idiots who take out a policy on a crappy car and flout the conditions of DOC by driving expensive and powerful cars.
Furthermore, if you insure a L reg Escort, but then drive a 55 plate Impreza under DOC and have an accident, there is a very good likliehood that you will end up liable for all costs associated with the claim, and possibly face police prosecution and points.
If you find the overwhelming need to drive another vehicle that is not owned by you, either become a named driver on the policy that covers that vehicle, put it on temporary cover with your insurers (and pay the premium), or take out another policy.