In the UK, passengers in the back seat must wear seatbelts as well as those in the front seat. The person liable for failure to wear seatbelts is the driver. You can be fined up to £500. https://www.gov.uk/seat-belts-law/overview

If a vehicle registration (V5 / logbook) states that a vehicle seats 4 passengers, you cannot seat any additional people. If you do and you get into an accident, your insurance is invalid! If you are caught by the police, you could get points against your license for overloading and a fine (offence code CU50, which incurs three points and a £60 fine) in addition to the penalties for no seatbelts.