I've looked at Senditnow too, but their maximum dimensions seem a bit tight too...or is this as small as I'd need to pack it down to?
"In terms of dimension, the longest side must be no more than 1.5m, and if you take the total measurement all the way round the thickest part of your parcel (we call this the girth) and add to the longest side, this must total no more than 3m."