Travelling in living accommodation
First, I agree with the don't do it answer.
Legally however it is a bag of worms.
Yes, current legislation says seat belts must be worn "if fitted" - no seat belts fitted - can't be worn. Depending on vehicle age then retro fitting may have been a requirement.
However as for sideways seats it is accepted that it is safer to be thrown violently around the inside of a vehicle than it is to be twisted umpteen different ways by a 'sideways' seatbelt.
Let's face it neither option sounds good.
Also remember that there is a limit as to how many passengers you can carry on your driving licence - this varies depending on when you passed the test.
However - overall I remain firmly of the opinion that it isn't a good idea even if it is actually legal.