https://www.diycontrols.com/t-automatic_water_shut_off_valves.aspx
Other than those the wondervalve is very good as it limits flow and is fairly cheap, plus you can fit different cartridges for the flow you need.
Second bes, but remember that imperial 1/2" 1/4" 1" etc pipe and fittings is the nominal bore diameter, not that of the pipe.
See http://www.spgspg.com/library/Pipe%20Thread%20NPT%20and%20BSPT%20fittings%20compatibility.pdf
Temporary cylinder somewhere with immersion to save on pipework.
Or, and easier (?), an electric instantaneous hot water heater, though it will be dodgy to get the right temperature with a shower.
Does leaving the fridge out of the rear socket circuit isolate that circuit from further trips?
It could be a faulty MCB on that circuit (not sure how RCD connected) and also, if you're plugging the fridge into the front via an extension reel these have been known to (somehow) isolate further...
If you're going to have UFH then I suggest block/beam flooring upstairs and wet system. We did this and installed two circuits for the new down/upstairs rooms.
It was a lot of extra work (including fetching screed up stairs in buckets!) but worth it I think.
We have an Ideal Concept downstairs which is very good, and was installed by me 3 years ago. At work however we have the same, and they are just awful. Will block if you look at them.
So I'd definitely look at the plumbing/outlet connections.
We also have a Roca Dama-n upstairs, which uses...
I have the Neo kit, and very happy with it. Wired so I can't comment on wireless.
I didn't install probes in the wet UFH, but I think I recall from the manual that the probe is just to stop electric UFH from overheating, i.e. it's not evaluated by the thermostat to be "smart" at all.
I've used 3mm steel plate in places, bought from a local steel supplier. Just ask for 50mm wide flat bar.
If you really wanted to, you could solder some g/y single to it, did this recently on a cover for a socket box full of wagos which was at the edge of a new stud wall.
Everything else...
We've just installed one. 250mm head, and you get very wet!
You can't scrimp on the hot water timing if you have a tank. "On all day" or a luke warm shower for the next person.
Noseall, you don't need battens with those tiles, you need pattens. Wooden overshoes to protect your shoes against effluent flowing in the street.
Patten spacing depends on whether you use left pattens, or right pattens on any particular row.