I've had a further look into it and I'm not sure actually. I think you might be right, if the seat is fitted with isofix, there's nothing in the manual saying that you can't do this, but I would send an email to Cosatto to check before you purchase it, as I would expect the seat is not designed to be rotated past the harnessed stage and it may be that it's considered too much wear and tear on the mechanism. I don't know how practical it would be as they get bigger/older, but it's certainly much cheaper than a special needs seat.
Chicco Seat4Fix is also worth a look. In fact I know Car Seat Libraries UK (FB/Website) have one of these - if you send a message to the lady who runs it she will probably be able to tell you and if she can't she will find out direct from Chicco.
Do you have a Smyths near you? They tend to have these kinds of seats in so you might be able to look in person.
You could also try the In Car Safety Centre for advice, via phone or email if they don't have a branch local to you. They are very good with finding the right seat for children with disabilities or special needs, and because they have experience with both the standard seats and the special needs seats, they'll be able to advise you better on whether it's suitable to use a rotating seat that goes up to Group 2/3 with the seatbelt like these.