Because you must have motor insurance to drive a vehicle on roads in the UK, then yes, all fleets are legally required to have a minimum of Third-Party insurance cover.
If you use cars or other types of vehicles as part of your business, it’s imperative that you take out at least the minimum level of cover on every company vehicle. You don’t have to take out a Fleet Insurance policy – you can choose to insure each motor individually.
If you have an accident and cause damage or injure someone, or a vehicle or an animal or property, then having third party insurance will cover you. However, it would not cover repair costs to your own vehicle.
If you don’t have the resources in your business to manage and juggle several different policies then taking out a single policy for all the vehicles in a fleet is the only way to ensure that they all have identical levels of cover and help to reduce your admin burden.