Firstly, I haven’t been convicted of anything!
I would like to know; I have a 5 door, x. I am also able to put the back seats down and put in some blankets and turn it into a bed. Which, in effect, turns my car into a two seater. Would it be legal to drive around with people lying down in the back?
If you only have people in the back whilst you are stationary I cannot see any reason why you should not be able to do this – as long as the changes that you make do not render the vehicle dangerous.
As long as you are not fundamentally changing the structure of the vehicle I cannot think of any offences that you would commit. Lots of people convert vans into campervans with a mattress in the back.
However if you remove the back seats it would be dangerous for people to travel in the back without a seat or whilst lying down and I think you would be accuse of driving a vehicle in a dangerous condition if you were to do that.