Gianni,
From what I understand, if a private bus operator is running a scheduled service and collecting fares from passengers, then he must have a Bus Passenger licence. However, if the service is a 'private hire' arrangement, for example, a once-off school day tour where he is not collecting fares, then a licence is not needed. Bus operators also need a Road Passenger Transport Operators licence when carrying passengers for reward (similar to a haulage licence required by hauliers). Bus Operators also operate services under the Rural Transport Initiative. The Dept. of Transport should be able to help you out with specific queries.