Founds this on US based site
"Q-How can I tell when my child has outgrown his car seat?
A-Children should ride in safety seats with a complete harness system as long as possible. Most seats with a harness fit children up to 40 pounds, but a tall, thin child may be too tall for a ?convertible? seat, the type that
faces either rearward or forward, before reaching the maximum weight. If the child?s shoulders are above thetop slots, try a combination child seat/booster with higher strap slots. The harness may be used up to 40-50 pounds, depending on the model. The harness is then removed, and the seat can be used as a belt-positioning booster up to 80 pounds or more.
Q-What about children who outgrow their safety seats before age four?
A-Most 2-year-olds and many 3-year-olds are too immature to sit still in a booster with a lap and shoulder belt, which allows them to lean forward or sideways. Parents should consider getting a larger seat or a tethered
harness system for younger or more active children who outgrow their safety seats.
Children who are under three or four years old or are very active may not stay put if they are not strapped in snugly. Booster seats do not work well for these children because they may lean forward or place the shoulder belt behind the back or under the arm. Special products that provide upper body restraint for children over 40 pounds are available, but they may be difficult to find or to install.
This is a US site but the same guidelines must apply. I will look for the UK version though.