This company is cheaper and free delivery for ProPlates. I have just ordered a set from them.
http://vanlockstore.co.uk/our-products/proplate/proplate.html
They're not all the same. Not obvious from the pictures in your link what you're getting, but you want the ones with an extended lower edge to the external plate, and an internal steel box that surrounds the lock mechanism.
Trade Van Locks aka ProtectaVan make good ones.