There are many places you can go to Halfords, John Lewis, Mothercare, and Kiddicare. They all have knowledgeable people who can help you pick the 'right' carseat for you.
Or you can go to the In Car Safety Centre. They only sell carseats. All their staff is trained in the proper installation and they have nearly every model on the market. It is a bit of a drive from the Cambridge area but we combined it with Ikea and made it a day out. You are advised to book an appointment so you just can't head out there. Also their prices are competitive.
The In Car Safety Centre is well worth the drive and expertise.
All the Best!
American to Britain