I think everyone is missing the point that you can only use the driving other cars extension of your policy if it is an emergency, not just because the car is available.
If you wish to use another vehicle, get added as a named driver - the police are clamping down on this (and rightly so), so if you're caught driving her vehicle and it's not a life or death situation, you're likely:
a) to be prosecuted for no insurance
b) have the vehicle seized
c) your girlfriend could be prosecuted fro allowing you to drive without insurance
And don;t forget that driving another vehicle under this part of your policy (assuming you have it) reduces cover for that vehicle to third party only, so you are only covered for any other party's damage i nthe event of an accident, and you wouldn't be covered for any fire or theft claims.