Many insurance companies no longer include 'any other vehicle' as standard because they were being ripped off by couples who would insure the cheap old banger belonging to her and drive round in his 'go faster' coupe without bothering to get it insured.
It must specify on the policy - and very often this is restricted to 'in an emergency'. The example the insurance company gave me was if the driver was taken ill whilst I was a passenger, I would be able to drive the driver home or to hospital.