ok, i think i understand. When you said baby car seat, i thought you meant a group 0 seat (i.e the ones for newborns) that you can just take in and out of the car at will. you need to ignore my forst three answers!!!
Britax have a fit finder section on the website where you can put your car in and see which ones fit.
http://www.britax.co.uk/car-seats/fit-finder If it is supposed to fit in your car, and it doesn't, take it to the shop you bought it from and either get them to do it, or get a refund