You must have a Driver Certificate of Professional Competence (Driver CPC) before you can drive a passenger-carrying vehicle (PCV) for a living.
There are four modules in the Driver CPC: two are the same as the tests you’ll take for getting your PCV licence.
1. Theory test: this is the same as the theory test for your PCV licence.
2. Driver CPC case study test
3. Driving ability test: this is the same as the practical test for your PCV licence.
4. Driver CPC demonstration test
You can find a guide to the Driver CPC on GOV.UK.