All I can suggest is that you try as many as possible. Mothercare are pretty good for this, Halfrauds... I guess you can try...
My daughter is now 4, so the next stage up from what you need fits fine. I went through this pain (and believe me, it was) when I was driving a GT, and that was bad enough. I also had a couple of 'show' minis that we used at weekends etc, so we needed seats that could fit easily in the back of those.
To cut a long story short, we found that isofix seats (ours, we had 2 britax ones) were physically smaller than standard ones, which helped a great deal in the minis, even though we didn't use the isofix clamps.
The main drawback here is the cost.
I'm sure you'll get many suggestions, (as you do when asking for opinions on anything frankly
) but really you need to go somewhere where you can try the seat for a fit.
You need to be careful to make sure the seat fits well, and that you don't get buckle crunch (EVERY seat we tried in the GT suffered from this, and was the main reason we went for ISOFIX seats in the end)
Oh, and if at all possible, I would try to avoid using the front seats.
Best of luck