The road traffic act states that it is illegal to use, allow to be used, or leave an uninsured vehicle on the public road.
Your partner would technically have been covered unde rhis own insurance, but this is DOC (Driving Other Cars) - and this does what it says on the tin - it covers you to drive other cars only.
I think you've been a bit naive here, and you're looking at an IN conviction (6pts, fine) - possibly IN14? Causing/permitting - not entirely sure which conviction covers this.
You need to lok at this from the other side of the fence - what if your vehicle rolled for whatever reason and killed a pedestrian and her 6 month year old baby? Granted the insurance would only compensate financially for the loss, but if you;re uninsured, you're in serious trouble