All children under 135cm or under 12 must use a child seat, whether in the front or rear of the car.
All child seats must be EU approved models.
Child seats must be rear facing for children under 15 months old or when the child weighs 9kg depending on the type of child seat, some are height bases, others are weight based.
It must be fitted using ISOFIX anchor points or seat belt anchor if designed to use seat belts.
Front air bags must be deactivated.
The law does change so whilst this is currently accurate they may well be new legislation introduced in the future. Check the gov.uk website for current details.