It looks as though it's made of a polyester satin (15% silk, 85% polyester) and is A-line in shape.
It looks as though it's made of 4 pieces plus the sleeves - a front, a back, and two side pieces with the majority of the shaping done with two seams on the front and the back of the dress. If I am honest I think the seams are having to work too hard and it looks like they're struggling - it would be better with more darts.
I wonder whether you could find an a-line satin long sleeve dress and then deal with the bow/sash once you had obtained the dress? If it were a longer dress than you needed you could potentially sort the bow from shortening the dress.
Otherwise I agree that I would look at getting it made, though I think £250 might be pushing it including materials, though TBF polyester satin isn't too expensive, though it looks fairly heavyweight.