Polystyrene or other foam instead of mineral wool in low pitch loft
You can put polystyrene between the joists or across above them.

The super-duper products for this do get you equivalent insulation to fibre wool at a lesser thickness but at a higher cost in £££s. Search for Celotex or Kingspan insulation boards. They are available at a wide range of thicknesses (25mm, 50mm, 75mm, 100mm). They should be cut to the width of the gap and friction-fitted. For maximum insulation at minimum thickness, you would use 100mm between the joists and another layer (your choice of thickness) above the joists.

To maintain storage space you should apply batons at right angles to the joists (installing insulation boards) and then board over the top with tongue and groove chipboard floorboards (use wood glue on the tongue/groove interface and wood screws into the batons). The required spacing of the batons will depend on the thickness of your chipboard - approx 400mm between centres for 18mm chipboard or 600mm centres for 22mm chipboard.

A good head torch and electric screwdriver will be invaluable.