So 5m3 of mortar. With density 1.6-2 and thermal capacity 800-1000 J.kg/°C if the slab heats by 1°C that means 1.7-2.7 kWh is stored.

So you can store a bit of heat in the slab during the cheap kWh hours, but probably not enough to keep the house warm for the rest of the day.

Slab reaction time should be a few hours, so if you have large south facing windows letting in plenty of sun you may find it useful to use a thermostat to stop the heat pump when the sun heats the house enough. Otherwise it'll keep heating the slab and a few hours later when that heat gets released it gets too hot.

Now I see you have exterior insulation: if the walls are heavy masonry (stone/brick/concrete not hollow blocks) and not insulated on the inside, this adds a huge amount of thermal mass, quite effective at smoothing temperature variations. In this case you can probably exploit the cheap kWh hours more to store heat in the masonry, also store free heat from the sun.

PE-RT Type 1

This doesn't say whether it has EVOH oxygen barrier or not.

9 circuits, at 10cm centre to centre, planned layout image attached to this message.

10 cm, you're going for ultra low temperature then! Good for optimizing heat pump COP, but there are a few gotchas.

For example, if you run 25°C flow and ambient temperature is 20°C then you can't have 5°C deltaT between flow and return, more like 3°C, which means for the same power you need higher flow.

If the pipes are long, at low flow temperature, and low flow rate, you can get into a situation where most of the heat has left the pipe way before reaching the end, which gives uneven heating. A way to fix that: in rooms with 2 loops, just run the 2 loops together, ie route two pipes side by side, but reverse the flow direction of one pipe, a bit like a countercurrent heat exchanger. This gives more uniform floor temperature.

100m ID 13mm PEX pipe gives Kv ~ 0.43 m3/h (that's the flow for a pressure drop of 1 bar). With manifold/flowmeter losses you get about 3 l/min in this circuit for a circulator-friendly 2.8mH2O (0.28 bar) pressure. With 9 circuits that gives 27 l/min (1.6 m3/h) which is absolutely fine for your future heat pump, it gives 9.5kW at deltaT 5 which means it'll have enough flow to run at a much lower deltaT on a 5kW heat pump.

Note I'm assuming it's balanced for the same flow in all circuits, which means the shorter ones need to have the balancing valve throttled on the manifold. Otherwise all the flow goes to the shorter circuits and the longer ones don't heat.

I'd turn the 2x 105m circuits into 3x 70m circuits, making them closer in length to the others. That should make balancing much easier and less fiddly.

With good insulation, I think one pipe every 10cm is a bit overkill: I have one 20mm PEX pipe every 15cm and flow temperature is something like 25°C lol.

I hear Samsung air to water heat pumps are a bit noisy, but never heard one myself. - that would not be a problem, as it's countryside

All heat pumps have noise specs which you can find.  Look for noise power not noise pressure, they are different but both measured in dBm.

So far as I can make out from this forum Samsung heat pumps are perfectly ok.  Their main downside is limited control capability.  If you just want to operate on pure weather compensation, which I suspect is appropriate for your situation, you are fine.  Forget night time setbacks or anything other than on/off adjustments  to suit tou tarrifs however. To do either of these other than by using an on/off temperature sensor, which is unlikely to be satisfactory in your setup, you will need homely as well as the Samsung controller unless you want to play around yourself with the modbus and home assistant or some do it yourself control electrics.

- Is it a slab? Or some of the other variants like pipes in aluminium diffuser plates? - slab

- If it's a slab, thickness and material? (concrete, anhydrite, etc) to get an idea of the thermal mass ; also what kind of floor covering? tile? - cement and sand mix, 7cm (5 for pipes + 2 on top), tiles all over

- Triple check the pipes in the UFH are either PEX with anti oxygen barrier or Multilayer Composite. Best way is to check the markings on the side of the pipes, if you have some left. Or just post pics. - PE-RT Type 1

- Number and length of circuits, to calculate pressure drop & flow (requires knowing type of pipe as 16mm PEX has 13mm internal diameter but MLC is 12mm). - 9 circuits, at 10cm centre to centre, planned layout image attached to this message. Again, I created that myself, aiming to keep each circuit pipe length under 120m as per recommendations. (PS: ignore the red square in Z3 area)

- More details to help understand your situation: what part of the installation is already built? is the UFH built? manifolds installed? pipes laid out to bring water to your UFH manifolds? if yes, type, diameter and length of these pipes? Do you visualize how the pipe layout will be, where to put the hot water tank, path for the pipes to the heat pump, etc, any foreseeable snags? Is it as simple as "well there's already a conduit so just push the pipe through" or a bit more "core drill through the whole house" kind of situation? - nope, nothing built right now, at the moment I only have the "main" slab to be covered with 10cm graphite XPS and pipe to be laid on top

I hear Samsung air to water heat pumps are a bit noisy, but never heard one myself. - that would not be a problem, as it's countryside

Thanks a lot for your time.

With a heat pump + UFH + good insulation, if the installation is not botched you're pretty much in the ideal case for maximum efficiency, lowest running cost, and excellent comfort.

About the underfloor heating:

- Is it a slab? Or some of the other variants like pipes in aluminium diffuser plates?

- If it's a slab, thickness and material? (concrete, anhydrite, etc) to get an idea of the thermal mass ; also what kind of floor covering? tile?

- Triple check the pipes in the UFH are either PEX with anti oxygen barrier or Multilayer Composite. Best way is to check the markings on the side of the pipes, if you have some left. Or just post pics.

- Number and length of circuits, to calculate pressure drop & flow (requires knowing type of pipe as 16mm PEX has 13mm internal diameter but MLC is 12mm).

- More details to help understand your situation: what part of the installation is already built? is the UFH built? manifolds installed? pipes laid out to bring water to your UFH manifolds? if yes, type, diameter and length of these pipes? Do you visualize how the pipe layout will be, where to put the hot water tank, path for the pipes to the heat pump, etc, any foreseeable snags? Is it as simple as "well there's already a conduit so just push the pipe through" or a bit more "core drill through the whole house" kind of situation?

I hear Samsung air to water heat pumps are a bit noisy, but never heard one myself.

@editor may be able to recommend an installer in your area, otherwise feel free to post details of quotes here and someone, not necessarily me, will doubtless provide some helpful comment.  Sadly there are cowboys out there, as is generally true in the construction industry so far as I can make out!  However if you exclude based on what I have said above you should avoid the problems that account for almost all the 'bad' installs we hear about here.