Cheap but not sure about the one key idea!
I'd check before buying, but I read it to mean you're buying X padlocks, all of which have the same key. I follow you in and buy another set and my locks will all have the same key, but different from yours.
That's how keyed-alike locks normally work, but I seriously doubt that Screwfix's stock handling system is capable of that. They're selling the locks individually, so if you buy X of them and they're all the same, I think all the others on the warehouse shelf must be the same as yours too.
As you say though, unlikely that a casual thief will have the key.
Thats crazy but I have just noticed the price
They rotate sets so that the liklihood of anyone having the same key as you is smaller. There should be anumber on your padlock to say which set it comes from. Less secure than other types but not really a problem since theves have easier methods of breaking a lock than carrying around a massive wad of keys in the hope that yours are from the same keyed alike set.