I just ordered some Rocksilk RS45 from
Insulation Direct 2 U. Search . I got 5 pieces of 2' x 4' x 4" and 10 pieces of 2' x 4' x 2" for about £66 delivered.
After searching around a while, I found that even with the £25 + VAT delivery charge, they worked out around the same price as shops I would have to drive an hour or more to.
Other suppliers have cheaper prices, but I found they had a minimum purchase threshold (£200 or £500, I saw) or just didn't have any of it. Other suppliers still have cheaper delivery charges, but much higher item prices.
I reckon you could find it cheaper, especially if you live near a supplier, but I found I didn't have to patience to search any longer. This company are based in the Birmingham area.