Segilla - what leads you to surmise that the agreement was verbal?
It is standard practice for a company to issue a document or handbook to employees allocated company vehicles. This handbook will specify the groups of persons authorised to drive by default and if necessary how to nominate others to be authorised. It is hardly practical to pass round the single insurance certificate to all relevant employees where a blanket policy is in operation.
Here is an extract from the Barclays Company Car policy in regard of Insurance...
"The BCC car is insured whilst it is being driven by you and any other driver authorised by you (hereafter ‘additional driver’). You may authorise an additional driver provided the additional driver is not disqualified from driving and holds a full driving licence valid in the UK. In all cases, an additional driver must have held a full driving licence for a minimum of 12 months before he/she may drive a BCC car or other vehicle provided by Barclays or GE.
http://www.barclaysco.../policycompanycar.htm