Is whitewood timber suitable for loft boarding?
If you can get the white board materials into the loft there there should be no reason that would prevent you from using them as a flooring material over your joists. Especially if all you are going to place on them are fairly light weight storage boxes and other sundry items. 

The relatively narrow boards may have a tendency to individually flex a small amount between joists if you were to walk around on them. Sometimes white boards can have sizeable knots that would span a good part of the width of the board. You would want to avoid installing board like that because they could flex enough at the knot to break the board.