One thing which I'd strongly suggest is looking at the "Which?" website (7.50 a month, but well worth it - plus you can cancel after 1 month). A lot of carseats you'd assume to be decent (big, high-street store ones) are very low on their extensive crash test findings. Some of the crash test dummy videos they have on there are pretty harrowing to watch.
The best ones seem to be the Maxi Cosi seats (especially for newborns - the Maxi Cosi CabrioFix came out as the safest car seat they tested).