By using short term car insurance UK, you can absolutely insure yourself safely and legally on a car you don't own.
The biggest benefit to this is that you only have to insure yourself on it for the time you need, so you can potentially save money by insuring only for the time you're driving.
Should you need to drive the car for a little longer than expected, then you can simply purchase another new temporary insurance policy and get to your destination, insured comprehensively.