Hi there,
I'm afraid I can't help with the first part but I can comment on the car-seat issue. I'm also in the UK and I had to look at this.

It seems that what you need is a 5 seat harness in the seat and that seat can be held by by a lap belt only. The seat also has to face forward, ie it's not one of the baby basket things that have a carrying handle.
Pretty much all of the seats I saw in the UK, from the cheapest Argos ones through to the fantastically expensive specialist baby shop ones, seem to match that requirement.

Jason