Yeah and vans have load securing lugs or reinfleced bolt holes to secure loads, they're there to be used! What law? You don't have to secure anything in a van of you don't want to.
Yes you do, there is a whole section in the road traffic act about it.
How do I know? Because I was once pulled over by a copper when I was transporting some equipment in an old van. He had a right go at me and was waving this big document (which was the road traffic act) around showing me the various regulations I was breaking. (He seemed to know it backwards, so be warned!)
Google "road traffic act securing loads" or something similar, you'll see there is a whole section and a code of practice too. The reason they take it so seriously is because you can become a real danger on the road if you get it wrong.
Thankfully he let me off with a warning and a stern talking to.