Honestly, in the UK there is no law that says a car restrain must be used, only that if one is available it must be used.
Cars - Children under 3 years old
If carried in the front seat, an appropriate child restraint MUST be used (the adult seat belt is not sufficient). Children under 3 years old may not travel in the front unless they are in a child restraint.
If carried in the rear seat, an appropriate child restraint MUST be used, IF AVAILABLE.
If an appropriate restraint is fitted in the front of the car, but not the rear, children under 3 years old MUST sit in the front and use that restraint. Remember, you could move the restraint from the front to the rear if you wish. Rearward-facing seats are designed to be used in the rear as well as the front. You should always put a rearward-facing baby seat in the rear if is a front passenger airbag fitted.
Cars - Children Aged 3-11 years (and under 1.5 metres tall)
If carried in the front seat, an appropriate child restraint MUST be worn IF AVAILABLE.
If not, an adult seat belt MUST be worn.
If carried in the rear seat, an appropriate child restraint MUST be worn IF AVAILABLE.
If not, an adult seat belt MUST be worn IF AVAILABLE.
If an appropriate restraint or seat belt is fitted in the front of the car, but not in the rear, children between 3 to 11 years old and under 1.5 metres tall MUST use that restraint or seat belt. (Remember, you could move the restraint from the front to the rear if you wish).
There are from a site with Government and ROSPA guidelines and legal requirements on:
Here