Hi, you should check out the Maxi Cosi Elea. It will work as a travel system and the pushchair frame folds small. So that it will fit in the boot of a cooper, the wheels also come off (via a quick release button). Unfortunately though, we are now selling our three door cooper and buying a countryman which will pretty much take any size of pushchair!
In terms of car seat, We purchased the maxi cosi family fix base which will work with the maxi cosi pebble upto 6 months. When baby is older we will buy the pearl which will be suitable from 6 months to 4 years.