Honestly I was doing for free as a volunteer, I was told I was covered and they have more than 3 Minibuses with different tons and they were not aware of this clause as well as myself

I have a category B driving licence for 8 years, I did not know that I cannot drive a minibus more than 3.5 tons, I was stopped by the police yesterday driving a charity minibus more than 3.5 tons. The police are charging me for driving without valid licence as well as invalid insurance but the charity organisation said there is a fleet insurance on the minibus. Please advise me