I would say not, most fully comp insurance premiums cover the main driver to drive another car under the insurance policy they have set up, usually only if the other car is insured and the cover is only 3rd party.
I got a fine for doing exactly as you state, i was driving a friends car as they were drunk but that made no difference to the judge.
Phone the brokers and as them directly, but it will probably say all that in the policy.