In a nutshell : you may do, but it’s certainly not guaranteed.
I can use my own car insurance policy to illustrate this. I’m the Policyholder, and my partner is the only Named Driver on the policy.
Here’s what my insurance schedule says that I can use the vehicle for :
And here’s what my partner is covered for :
So in this case if my partner borrows my car then they have cover for Social, Domestic, Pleasure and Commuting to a single place of work – but they don’t have cover for any other business visits they may make.
This can be a common cause of uninsured driving – if my partner borrowed my car (say whilst their vehicle was being serviced) then they’d be covered for their commute to and from their normal office but they WOULDN’T be covered if they drove to a training course, or to visit a client, or even to drop a letter off at the Post Office on their way home!