You're just not looking quite hard enough then. When I click on their "Shop By Brand" Velux category, that has a bit of text, well written, title and meta description are decent, then you click past that to Velux Centre-Pivot Roof Windows and once again, more content, mostly text.
You get into the actual product pages at some point, and they include a decent content set, but upon first glance you'll notice that most of that content is duplicated between "like" products. Have no worries though, because they are using the canonical tag to point all "similar" products back to a single product of that type, so that duplicate content doesn't war against itself. It looks like from every category page that includes products, they are pointing all of the products canonical tag back to what I assume is the most popular size.
So where you're seeing content that isn't great, I'm seeing enough content that is "good enough" to be considered great due to the quantity, and the organization of it. Not to mention, it seems like behind the scenes they've actually tried very hard to make sure every page is geared towards targeting specific terms. Therefore, they aren't spreading themselves thin.
MORE IMPORTANTLY, you ask, what are they doing that is so great compared to the others (like Wickes and so on), and I see them doing a lot better. But you can't just look at the site. You have to dig into their source code. For instance, Wickes is allowing their site to eat itself stupidly by having similar pages pointing to themselves via the canonical tag, as opposed to pointing identical products (other than in size) to a single item like the site you mention does.
I compete in a similar "home improvement" industry and we are up against sites like this all the time. Our issue is that we cover a specific "niche" within this industry and many of our competitors do it all, and we're constantly battling their "quantity of decent/ok content" vs. our "quality and more focused content".
Like you, we also have a superior link profile to most of our competitors, but I often assume it's their content that gets us. We tried to offset that by being the only company in our industry with a blog, but we just ended up cannibalizing ourselves to the point that I think we'll have to ditch it soon.