Found in Home Bargains Milngavie for £3.99, not sure if national. Quick search on Google and the cheapest I've found them is £12.99.
ABOUT THIS ITEM
100% Officially licensed Harry Potter Merchandise
Features an additional compartment to store your laptop, tablet or any flat device
Not suitable for ages 3 and under
Harry Potter kids printed backpack 44 cm, 35 Liters, Brown.