They use a standard Blackburn rear rack which gives you full failiure redundancy as it is 4 point mounting (not 2 point like most others)
The Taxi model is £80 and has a slide and lock fitting, 3 point harness and foot straps to stop you getting kicked in the rear. it even has a fail safe lanyard.
The Limo is around £100 and can be reclined. It can also be stood on the floor on its own..
very useful if you are picnicking as little one can sit in the seat while you stuff jam sandwiches in its face.
Both have strong/high impact resistant sides to help protect the little one and will carry upto 40 lbs in weight (that's a small 6 year old by my calculations).
All the child seats I've seen here (UK) say "from 9 months" or "when the child can sit upright unaided for the duration of the ride" - in the USA its 12 months.
The issue is the neck muscles aren't strong enough to hold the heavy head and the child could get a neck injury. I've not found much info on the net but there's plenty on whiplash and car child seats.