In the UK, car insurance is linked to the driver rather than the car.
As a standard annual policyholder you may be legally covered to drive other people's cars in the UK. However, in the vast majority of cases this is only third party cover (i.e. the minimum basic cover required by UK law), so if the accident is your fault you will not be covered for damage to the vehicle you are driving.
There are a couple of ways to become fully comprehensively insured:
Option 1: Contact your current insurer and explain what you want to do. You will need your licence details and your credit card and this is likely to cost a significant amount.
Option 2: Get short-term car insurance with Cuvva. All you need to do is enter the licence plate, tap, tap, tap and go.