I didn't know there was a difference. We have a Graco car seat which came with our travel system. Graco is a US company. It has two straps that come down over the shoulders and join together then slot into a third one which comes up from under the crotch area. According to the instructions that came with it, where and how it is situated in the car depends on the car model. For some, it recommends rear facing, others forward, some in the middle of the back seat, others behind either the passenger or driver's seat.
I can only assume any differences between the UK and US car setas must be due to the different safety regulations between the US and EU.
Pebbles might know more, she has experience with babies in both the US & UK.