The rules regarding who can drive a minibus in the UK are determined mainly by when the intended driver first passed their car driving test and also if the driver is being paid to drive the minibus.
If the entitlement to drive cars was given after 01/01/97 then the driver will need to apply for a minibus licence (category D1). There are various exceptions to this pertaining to Minibus and Community Bus Permits and also the issue of hire and reward and whether the driver is being financially rewarded (paid) for the driving of the minibus.
If the entitlement to drive cars was given before 01/01/97 then the drivers licence will show either category A (manual cars) or B (automatic cars) and D1 which allows for the driving of a minibus (with minimum of 9 and a maximum of 16 passenger seats) not for hire and reward.
It is recommended that drivers licences are checked at least every 6 months to ensure that they are still valid as categories can be removed by the authority.