Hi John,
I thought it would be abundantly clear within your policy, what you are covered or not covered for. Your policy is either for you and named only, no third party coverage. You only fully comp, then third party only for other vehicles. I would have no idea how they would police whether coverage is for your partner, casual girlfriend, cousin etc.
Unless you are mental and live with someone and are happy to drive around in their vehicle thrid party only. That would be madness compared to being named on the policy for a car that you are obviously going to drive regularly.
H.