Well, a couple of things occured to me after I was MNing this afternoon.
Firstly, one way of getting round the problem might be to install the carseat in the front passenger seat (assuming that you can safely disable the front passenger airbag). This is apparently common practice in Scandinavia. It might be acceptable to you if it is generally you and your DS in the car with only occasional trips with 3 occupants.
Secondly, if you are both tall (like ourselves) you might have to wait until you change cars to get a RF seat - if so, look on the rearfacing website forum, I posted a list of cars that are more likely to take RF seats and tall adults. These included some vehicles that were not prohibitively expensive, as in some cases, slightly older models of vehicles allowed more legroom for the passengers front & back.
To get an idea of whether you could install the seat in the rear and still fit comfortably and safely in the front passenger seat, you need to position the seat where it would need to be, then measure the shortest distance horizontally from the back of that seat to the back of the seat where the Be Safe seat would be installed. If the distance is 76cm or greater, you may be able to fit it all in, but I have no idea how big a Ford KA is, and from what Cathy said this afternoon, it is unlikely to be a goer.
At the end of the day, you might have to use a FF seat until you can upgrade your car, and then perhaps look at the seat again when you make that purchase. FF seats are pretty cheap, so it wouldn't be a huge loss financially, but I can understand if you are less than happy with that in terms of safety for your DS, as I felt pretty much the same frustration when we were looking for seats and couldn't find one which would allow me in the car at the same time!
Anyway, let me know how you get on and I'll ask DH to investigate further if you need him to.