I know you posted this about a month ago but if you are still looking to get a box the right size i have a idea for you.
I work in ikea here in Dublin and in my department we got a machine around 2 years ago that cuts the boxes for all our products when they get ripped out on the shelf. Most stores i know of in england have one as well, if there is a ikea near you head on in and go to the "AS/IS" / "bargain corner" area, that is were it will be in the back. the department will be called "recovery" ask if they will cut you a box and give them the dimensions.