This website is very good for giving you the law and information, for the Uk, about use of car seats and restraints.
The UK Law
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 to 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).
Cars - Children Aged 12 or 13 years or younger children 1.5 metres or more in height
If carried in the front or rear seat, an adult seat belt MUST be worn IF AVAILABLE.
Cars - Passengers Over 14 years old
If travelling in the front or rear seat, an adult seat belt MUST be worn IF AVAILABLE.