As from the Monday after next (18th September), children can travel in the front passenger seat of a car, using the adult seat belt, from their 12th birthday or from when they reach 135cm (approx 4' 5") in height (whichever comes first).
Prior to reaching the required age or height to use the adult seat belt, children will still be permitted to travel in the front passenger seat as long as they are using the correct type of child restraint. This is defined by the child's weight and not by their age or height. For details of the type of child restraint which should be used for a specific weight, see Section 3 here:
http://www.thinkroadsafety.gov.uk/campaigns/ch ildcarseats/childcarseats.htm
Chris