Just come back from our holiday and thought it would be useful to post on here re using a car seat on aircraft as we purchased a seat for our 13 month old daughter. We flew with Easyjet and purchased the Britax Eclipse and it fitted perfectly on the Easyjet seat. The dimensions for the width were 2cm more than Easyjet stipulated, however, the widest point of the seat was well above the armrests. All that worrying over nothing!
I researched so much prior to our holiday to find a seat that would fit the dimensions and found it quite difficult, so I thought it might help anyone else trying to find a suitable seat.
Tip : I also purchased the aircraft fitting kit, which cost £8. It was a waste of money really as the approval sticker was on the car seat already and we didn't need the belt shortening buckle, but I suppose it gave me peace of mind. Also, we fitted the seat ourselves (which we expected anyway), so make sure you check the fitting instructions before you go.