UK law requires children to use a car seat until they're 12 years old or 135cm tall, whichever comes first. But the legal minimum and the safest option aren't always the same thing.
The law
Children must use the correct child car seat until they're 12 or 135cm. After that, they must wear a seatbelt. It's the driver's legal responsibility to ensure children under 14 are in the correct restraint.
Rear-facing is safest for longest
UK law allows forward-facing from 9kg (around 9 months), but safety experts recommend keeping children rear-facing as long as possible - ideally until at least four years old. In a frontal collision, rear-facing seats distribute force across the back and head rather than concentrating it on the neck.
i-Size (R129) regulations
The newer i-Size standard uses height rather than weight to determine seat suitability, and requires rear-facing until at least 15 months. i-Size seats must be used with ISOFIX points.
Common mistakes
Bulky coats under harness straps (compress the coat, put it on backwards over the straps, or use a blanket). Loose harness straps (you shouldn't be able to pinch a fold of webbing). Moving to the next seat too early. Second-hand seats of unknown history (internal damage isn't always visible).