I was considering ordering that exact same model and managed to sort a half day test drive (Didn’t tell the I was a Motability customer), but after a couple of hours of driving I returned the car as my back was in agony. I had tried altering the seat in every way possible, but to no avail. My wife felt exactly the same and couldn’t wait to get out of the car.
The seats are the same sports ones that are used in the 1 series and are very firm, and there is no other option. I would spend as much time as possible in the seat to make sure it’s ok for you.