I drive a jeep wrangler -- small boxy suv with removable hard top. It's a new model, so it has places to anchor a car seat in the back. New car seats, newer cars have a Latch system, so you don't have to use seatbelts, but ties which attach to the hooks on the floor of the trunk or back part of the vehicle. So for the first year DS was in graco car seat in the middle of my car -- the base stays into the car and the carseat carrier latches into it -- we bought an extra base for DH's car. Then as he got bigger -- a year old I got an evenflow triumph. It's like a big old easy chair. It actually has reclining cabability.
Only problem with placing him in the middle of a 4 seater car is that anyone who road with me in the back seat, next to him had to be semi-skinny.
In any event I believe Britax makes car seats for the UK. Rather expensive and some people with smaller cars have had difficulty with them.