Happy to be corrected, but I think it all depends on when you passed your car test; pre 1997 you would have grandfather rights. After that date you’d have to take a test.
BUT… to drive it for hire or reward you need the appropriate DCPC if you’re carrying passengers, and it’s a different animal for buses. There are certain exemptions for charities and the like, but the safe bet is that if you’re being paid for it you need the licence and the DCPC.
In a nutshell, if it ain’t on the back of your licence you ain’t got the entitlement.