The best advice I can offer is to contact your insurance company.
They will normally be happy to tell you if you have cover to drive another vehicle.
My own fully comp policy allows the use of any car that is not owned or hired to me.
Stricltly on a third party only basis. ( Saga insurance )
My fully comp commercial policy, provides no cover to drive any other vehicles. ( Direct line )
My 20 year old sons fully comp policy, does not provide cover to drive other vehicles untill he reaches 25
It varies from company to company. If in doubt. Check with you insurer
