You need an operator's licence to operate vehicles above 3.5 tonnes gross vehicle weight (gvw) that are used to carry goods (i.e. anything not permanently attached to the vehicle) on public roads for trade or business purposes. This includes short-term rental vehicles hired for as little as one day.
The operator's licence must be held by the person – whether an individual or a company – who ‘uses’ the vehicle and this may or may not be the owner of the vehicle. The user of the vehicle can be:
the driver, if they own it or if they are leasing, buying on hire purchase terms, hiring or borrowing the vehicle (eg a typical owner-driver operation)
the person whose servant or agent the driver is – i.e. whoever employs or controls the driver