I've no idea about Asos but I've shopped at B&Q a bit and never seen that and I can't imagine to understand how Argos would do it with their business model.
I was speaking generally, not about those specific stores. Of course, it only took a minute to find something on B&Q which was in selected stores only, not online 😀
On the contrary, it's quite common. Promotional pricing often says either "Online only" or "In store only"