The law requires all children travelling in the front or rear seat of any car, van or goods vehicle must use the correct child car seat until they are either 135 cm in height or 12 years old (which ever they reach first). After this they must use an adult seat belt. There are very few exceptions.
It is the driver's responsibility to ensure that children under the age of 14 years are restrained correctly in accordance with the law.
The law is different for buses, coaches and minibuses with seat belts fitted.
Types of seats available for different weights of children can be found here: http://www.childcarseats.org.uk/types-of-seat/
Web address: www.childcarseats.org.uk