Some fully comprehensive policies will cover you to drive other vehicles (in addition to your own). But before jumping behind a different set of wheels, check with your insurer that you definitely have this extended cover. You might also be covered to drive hire cars too.
However, it's worth noting that even if your comprehensive cover allows you to drive other vehicles, this is usually only up to the level of third-party coverage – so no theft or fire damage cover is provided.
If you share a car (or someone else regularly uses your car), you can add other drivers to your policy – known as named drivers. Sometimes adding a named driver can even help to lower the cost of your own cover, especially if they are seen as 'responsible'.