Yes, well, our baby girl Emma is four weeks old now, and this is the first chance I've had to check this thread since, so now is a good time to report our findings!
We went for the Cameleon / CabrioFix combination in the end, with the additional EasyFix ISOFIX base. So far, we absolutely love it! I can confirm that, on our tintop Cooper, the carseat does indeed fit nicely through the gap between the back of the passenger seat and the door frame, as long as the seat is pulled a fair way forward. Once you get the hang of it, the carseat clunks nicely into the base, settling firmly and easily into position with a minimum of fuss. Emma seems pretty happy with it, and the passenger seat is perfectly usable afterwards, as long as your partner is not a pro basketball player.
The folded Cameleon chassis sits nicely in the boot, but you do need to remove the parcel shelf and one of the large wheels so that the tailgate closes. However, as Timble states, this takes a couple of seconds to do with one hand, the Cameleon is such a well-designed and conceived bit of kit. It's even possible to stash a few Tesco bags in the space in and around the chassis once it's in there too. Timble also makes a good point about the MaxiCosi adaptors for the Cameleon.... they are quite easy to lose track of! We try and make sure that we always store them in the boot or in the carseat itself when it's not in use.
So on our outings thus far, I've been driving, with Kelly (who can't drive for another two weeks because of her Caesarean) in the seat behind me keeping an eye on Emma, who's in the carseat behind the passenger seat. Bugaboo goes in the boot, shopping in the remainder of the boot and in the passenger footwell. Very, very happy indeed with all the kit, it works very well. Not sure how easy it's gonna be lifting Emma in and out in her carseat when she puts a bit more weight on, tho.... she's only 8lb at the
mo!
(As a little postscript, by far the most impressive bit of baby transport gear we've found so far, that makes it much more bearable having a small car, is the Baby Bjorn Active carrier. Just chuck that in the boot when you go out shopping, you don't even need the Cameleon chassis! We just park up, leave the carseat in its base, pop Emma in the Bjorn and she's happy as Larry. Highly recommended.)