Not all Fully Comprehensive policies offer DOC (Driving Other Cars) coverage. It is entirely at the discretion of the insurer if they wish to include this clause in the policy so it is prudent to contact your insurer and ask them directly if they do offer this and under what conditions (emergency use only etc).
DOC usually only covers the car TPFT while you are actually driving it. Once you leave the vehicle it no longer has insurance coverage, rendering it illegal if left in a public place.