i think you'll have to explain your question a bit more. Different makes and models have different numbers of seats. Putting 3 people in a smart car would be dangerous. Putting 3 people in a citoreon picasso wouldn't
"it would be illegal to drive people who are not belted"
No it wouldn't. the law states that a seatbelt must be worn *if one is available.*
You can't carry more than 8 passengers on a standard UK driving license, you would also need to check the insurance as most insurance policies have a limit on the amount of people that can be carried that is based on the car.
Wow.... they really have made the minibus entitlement a minefield for drivers that have passed after 1 Jan 1997!
If a driving test was passed after 1 Jan 1997 you won't have D1 entitlement on your license, but you can still drive a D1 as long as certain condition are met!