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Have no idea on that particular rule. The BCO didn't have objections, probably because the tarmac isn't being left under the house. I have other uses for it making some pathways in my woodland. The underfloor area is getting dug out by 4 inches (so I get 300mm air gap to the floor beams) and covered with a membrane and gravel. My BCO is quite insistent that I give the vegetation no chances. And since the local brambles seem to be cross bred with Japanese knot weed I will follow the advice! This was all going to be in the next chapter should it ever stop raining for more than a day here.

On an UVC always, but, you can get away without one on primary circuits as it’s just the volume of expansion to get rid of. BCO’s never chase on that but are all over D1 & D2 on potable as it’s drinking quality water being ‘lost’. There are several waterless traps ( sealed ) which are transparent, so you can use one of those if a visible discharge is required internally. Where it runs to an outside wall you can rely on that as means of visible discharge.

You don’t really need the tundish as it’s not potable. Change it to a waterless trap if you’re getting draughts coming up

TBH, I think you're pretty well covered with the thermostat and the back up safety cut-out. Any additional safety cut-out you add would need to be as least as well-proven as the certified one in the immersion to be useful, I think. I can only once recall having an immersion "run away" and boil a tank, and that was over 40 years ago, before they were fitted with safety over-temperature cut outs. My experience since has been that, if anything, the safety cut-outs tend to operate when there isn't really a problem, as they seem to err on the side of being safe.