Staring at the price of the Sevenlife 18 latex mattress in the checkout window, I decided to hone in my Google-Fu to further uncover any additional information and options. The first thing I found was an Amazon listing, and while the vast majority of reviews were positive, there was one 1-star review (from almost 6 years ago) that stood out:

My whole justification for such a £600 mattress was that it would outlast other options. A 10-year warranty is meaningless if it's financially infeasible to make a claim.

Understanding that the main issue is overseas shipping of a large and heavy item, I tried a few searches in both metric and imperial measurements and found a company called Purelybeds selling mattresses in 1/2 ft increments up to 7.5 feet (228.6 cm):

Because I'd have some spare slats from the headboards, in theory, I could spend about £10 on a staple gun to staple the extra 2 - 3 slats for their longest option.

I continued to look for "7 ft" and longer mattresses and found Bed Cloud, a company that makes mattresses in 1/4 ft increments up to 7 feet at prices much closer to those found on Amazon:

The first one I clicked on was the one with the grid pattern because I liked the look, and it just so happened to be their 18cm soft/medium offering which is very comparable to the Sevenlife 18 with the main difference being open-coil vs latex.Since the monitor is being mounted on the frame instead of standing on a pane of glass on-top of the mattress, I believe the non-localised nature of open-coil shouldn't be an issue.

Looking through the others shows a trend that higher-cost mattresses are firmer. I was able to find reviews on Amazon and, like the Sevenlife, most of them are positive. One gave a 4-star because it was softer than they'd like, which sounds good for me since I feel that "medium" mattresses are too hard.

It took a bit of effort to rework the frame to use a Three Way Outlet instead of a Three Way Through to be able to shave off the 7 cm I needed, in part due to Fusion sketch solving issues I had to track down and resolve. There is a minimum 100mm tube cutting length, so that's why there's a vertical 75mm and horizontal 50mm tube instead of them being shorter. 

The price of the frame increases by around £12, now £549, but the mattress savings can entirely pay for a new monitor and still have hundreds of pounds left over.

[Sep 1] 

I got the mattress maybe a month ago and the advertised "medium-soft" is indeed as firm as I can go. Furthermore, I was confused why "it feels so firm!... yet I don't want it any softer!", so that Dunelm technology was on point. I remember that the first night or two had my back feeling like it was being realigned in real time. 

While the mattress isn't made with pocket springs, deformation is much more localised than comparison videos had me believe. The only drawback is there are 2 metal wires that run down the length, approximately +/- 15cm from the centre. That's kind of the only telltale sign it's not fabricated like my old Latex Edge 2000. 

Would I appreciate another 15cm? Yes, primarily for lying on my stomach. Is that 15cm worth hundreds of pounds? Certainly not.