As far as i understand it...
From 6 May 2006 any child under 10 years old AND under 135 cm tall will HAVE to be in an appropriate child seat
The advice is up to 11 years / 36kg / 150cm (once 1 of those is reached you can stop rather than all!) BUT for the purpose of the law it will be all children under 10 AND under 135cm
It is recommended that children stay in car seats with a 5 point harness until they reach 18kg (as these are the SAFEST seats to travel in)
From 15kg they can go in a high backed booster seat (also a 'normal' booster, but this is NOT recommended until they reach 22kg for safety reaons)
there are seats that use an adult strap when less than 15kg (eg Britax ranger, Britax Crusier plus) but the WHICH? magazine found that these seats offer very poor protection (only the legal minimum) and therefore includes them in a section of CARSEATS TO BE AVOIDED!!!
Interestingly the Britax trio (which can be a harness seat, high backed booster and a booster cushion) also did VERY poorly and was in the seats to be avoided section.
Britax also do a seat Britax highliner which is a high backed booster seat which offers EXCELLENT protection and can be used until child is 36kg / 11 and offers much more protection than a booster cushion alone
(I personally am obsessed with car seat safety!!) I am a nanny with no children of my own yet i own 5 car seats cause im paranoid that any children i carry sit in the safest seats available!
I wont use the parents car seats, only my own cause then i know for SURE that they do fit safely!
Also My charges parenst will willingly provide the Graco Junior plus 9for me to use fro my charges friends), but becasue of its very poor review i just last weekend spent £55 on a Britax Highliner... just because it worried me on the rare occasion i had another child that the seat they were in was not safe enough....
My theory is, if ever i was in an accident and (god forbid) the children were hurt.... , by ensuring i only let them travel in the safest car seats i can buy, i could sleep at night knowing deep in my heart i had done my best for them!
Also IF you are in a crash, its then you want to be in the safest car seat!!!
So as a rule...
proper car seat with harness until AT LEAST 15 kg (max 18kg)
High backed booster until AT LEAST 22kg
Booster until 36kg (or 150cm)
It is WEIGHT that is the main factor, but childs height for example may mean a tall light child outgrows a group 1 car seat before they reach 18kg.