UK law requires that anyone who drives a motor vehicle has to have 'third party' insurance. If you hire a car, insurance will be included. If you buy your own car, you must obtain insurance prior to driving it.
You can drive someone elses car as a named driver if they include you on their policy or have "open driving", you must make sure they make their insurer aware of your licence situation, although your licence will be fine to drive with they have to be told or they could refuse to pay in the event of a claim