According to the official www.gov.uk site, a child must use a child car seat till they are 12 years old or reach the height of 135cm, whichever comes first.
All children under the age of three years, when travelling in any vehicle must have the right child restraint on. With the exception in case of an emergency short trip or in a taxi where a child seat is not available.
In the case of cars fitted with passenger side or front active air bags they must not keep the child in the rear facing position in the front seat.
Since May 2008 child seats must be approved to UN ECE REG 44.03 standard. You can read more about the Child Seat UN laws here. (This is usually market by a “E” label on the child car seats).
The seats should also meet the new ECE Regulation 129 (i-Size).
It is always the drivers responsibility to ensure that children under the age of 14 years are properly and securely restrained according to law.
The law for Taxis, Good Vehicles and Vans are different. You could find out more about Other Vehicles from Buses to Mini-coaches and camping trailers at www.childcarseats.org.uk