My Hammonds bed came with such slats included, its basically aeshthetic over comfort getting a wooden frame.
Looks great but needs a lot of stuff on top:
The 20cm Octaspring mattress was still extremely firm and uncomfortable on top of that, at the time I bought it I found a king size S Plus for £405, and the topper was £225.
Getting a dual layer octaspring mattress instead was over £1000, so that seemed like a good deal.
The 30cm triple layer octaspring things are stupidly priced, it seemed better to get a single layer one plus a topper, but it seems they soften a lot after a few years of use.
They do have a really long warranty on them, but I don't think that covers the mattress softening as that would be normal wear and tear.
I've used it firm side up since I got it, the soft side felt like instant back crack, too much sinkage: