(1) It shall not be lawful for any person (hereinafter referred to as the said driver) at any time to drive in a public place a mechanically propelled vehicle unless either a vehicle insurer, a vehicle guarantor, or an exempted person would be legally liable for injury caused by the negligent driving of such vehicle at that time or there is in force at that time either -
(a) an approved policy of insurance whereby the said driver or some other person who would be legally liable for injury caused by the negligent driving of such vehicle at that time by the said driver is insured against all sums without limit (save as is hereinafter otherwise provided) which the said driver or his personal representative or such other person or his personal representative (as the case may be) shall become liable to pay to any person (other than the excepted persons as defined in this Part of this Act) by way of damages or costs on account of injury to person or property occasioned by the negligent