Do these 2 x 4’s provide structural support?
I'd leave them in place and add a few more 50mm x 50mm (2" x 2") at 600mm (24") centers from the originals, thicken out the insulation to come up to the top of them and then plywood out (assuming the headroom isn't disastrous...). 

That way everything is happy and you save some heat loss...

Plus, if they are nailed in two directions, (into the ceiling joists and into the vertical bearer) they will be very difficult to remove and you could easily damage the ceiling below...