Thanks. My reckoning is that say a vented thermal store with mixer is (about) £300 more than a vented cylinder, then if the Horne 25 is (say) £150, then the former option looks more affordable. And I get mains-pressure hot water!
I'm not sure from your post what you plan to do but....
1. I'd be careful with planning to use a TMV with mains pressure only on one side. They will work with different pressures, but there's a limit to the differential.
2. Personally, I'm not sure what the advantage of mains pressure hot water is; I've got this dislike of high pressure showers due to the waste of hot water and energy, just use a good shower head on a low pressure system for a good shower and saving energy.