From what I've seen online kids are in boosters at 40lb.
Currently my dd is 33lb/39".
I think we do have children in boosters much younger over here. As far as I'm aware there are very few options for keeping little ones in a harness after 18kg/40lb. I have come across a couple of special needs seats but nothing readily available on the high street. I've been told (but not confirmed it) that our test dummies are 40lbs so nothing is approved over that weight.
A few places are starting to sell extended rear facing seats, they are available on-line and I think a couple of high street stores now stock them. I'm not to well up on that as mine are 8 and 6 so we're a bit past that stage. However when they were smaller forward facing at 6 months was pretty common, though we were told to keep them rear facing till the limits of the seat.
There is defiantly (around me anyway) a culture of moving on to the next seat as soon as possible and by preschool age most kids I know have been in boosters. I think in part down to physical sizes of the seats, which are quite small. I know DS at 3 was squashed in his seat but still way under the weight limit, he got another 18 months or so out of a combination seat before he reached the weight limit we had to stop using the straps.
http://www.childcarseats.org.uk/law/index.htm has a round up of the laws, and a bit about the different types of seat available.