Hi kezwald,
Welcome to the Mattress Forum! 
As you’re aware, this site focuses upon North America, so I’m sorry I can’t be of much help with souring materials in the UK. But I can comment upon some of your DIY questions.
The first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
As for your configurations, the kPa and ILD numbers for the density of Dunlop latex you are providing don’t seem to align too well, unless these numbers were achieved at a deeper 40% or even 60% deflection (versus the “usual” 25% deflection) using ISO 2439. Density is the best way to relate softness and one piece of latex to another in Dunlop latex, and most manufacturers would consider a 65 kg/M3 Dunlop latex to be in the “soft” range. The ILD and kPA you show is that of a firmer piece of latex. A kPa of 4.5 or so is generally consider to be firm for Dunlop, as would a 35 ILD. So the first thing I would mention is that you are considering using some pieces of “softer” Dunlop latex in your proposed construction.
Next, I would tell you to find out the actual density of the polyfoam cores you are considering using. The term “HD” for polyfoam is overused for a very wide level of foam densities, so you’d want to be given a number instead of a term for this foam. I’d recommend at least 1.8 lb/ft3 density.
As for how this would feel, that is the one thing that I wouldn’t be able to relate to you, as there are far too many personal variables and preferences involved, and unless you are replicating a mattress that you have already sampled in person, the only true way to tell how something like this would feel would be through your own personal testing.
Assuming the polyfoam core was at least a 1.8 lb density, the materials you are considering using certainly would be of a better and more durable quality range, and I would think that your resulting mattress would be in the “soft to medium” range, but that again is entirely subjective.
All of this is part of the DIY process, and if you aren’t able to copy a similar design that you’ve already sampled, then your best option is to acquire the guidance of a foam supplier or manufacturer who has experience with the type of componentry you are considering using.
Best of luck, and I’ll be interested in learning about what you decide to choose, or if you have more specific questions.
Phoenix