The house is reasonably well insulated. Average annual energy consumption for heating the house, based on my experience with the wood burner is ~16500 KWh. Worst day average this season is about 8.75 KWh. It is a big house with 240m2 of living surface.
Those numbers don't add up. Assuming that you mean kWh (kilowatt-hours) rather than KWh (kelvin-watt-hours), 8.75 kWh per day every day would add up to only 3194 kWh per year.
My issue with it is that we have a 9KVA mains supply and the "Linky", our intelligent electricity meter, might cut us off when the current surpasses the 45A it allows. With a 2KWh water heater, a refrigerator, a freezer and the heat pump possibly running at the same time might be enough to do this.
... or are you possibly using kWh (unit of energy) when you mean kW (unit of power)?
For one, you could use some sort of lock-out mechanism to stop heating while the water heater is active.
But also: Have you looked into using the heat pump for your hot water as well? After all, if your heat pump can reach the necessary temperatures anyway, it shouldn't take that much to have it heat your hot water as well, which generally should roughly halve the electricity you need for that.
Also mind you that most heat pumps are modulating, so even a nominal 9 kW compressor will not run anywhere near that most of the time. My own air/water heat pump is a nominal 4 kW model, but it only pulls ~ 3kW while heating up hot water, otherwise, it runs at ~ 300 to 500 W down to ~ 0 °C outdoor temperature with a few minutes of ~ 1.5 kW after startup. So chances are you'd only have to deactivate the water heater when it's extremely cold outside to stay under the limit. On the other hand, if you plan with resistive backup heat, you have to consider that power in addition to the "normal load".
As you mentioned that you required relatively high water temperature for your radiators: What is your water throughput and temperature spread with the current setup? Heat pumps are generally most efficient with the lowest-possible water temperatures, and as a consequence of that are generally designed to run at high throughput and thus low temperature spreads. So, if the currently required high source temperature is just a result of a low-throughput pump, you might be able to improve things there. But if the pipes are too small, that also can get you a noise problem, of course.
Also, beware of storage tanks and the like with heat pumps. While there might be uses for them, they are also often used by contractors to hide bad planning and end up reducing efficiency. Especially, they tend to be used to hide a mismatch between an oversized heat pump and the house where the mismatch would cause the heat pump to cycle rapidly, and the storage tank is used to absorb more heat faster and then slowly release it to the house. But that necessarily means that the heat pump runs at higher temperatures than necessary, and that is bad for COP. Ideally, you want your heat pump to be running 24/7 at the minimum output power that is required to keep your house warm.
In the same vein, beware of oversized heat pumps in general. Contractors tend to recommend oversized heat pumps because they want to be sure the customer won't complain about lack of warmth. And also because of incompetence (because oversizing largely didn't hurt with fossil heating systems and that's all they really know). There is a limit to how far a heat pump can modulate down (around 50%, varies by model, obviously), plus it might be a bit less efficient at the very low end, and if you are using a heat pump that's nominally oversized by a factor of 2, you will effectively not be modulating at all because the minimum power is sufficient for the coldest days.
As for your house being split between floor heating and radiators: I have no idea whether there are any off-the-shelf solutions for this, but maybe it is possible to run the heating in some sort of interleaved mode? Like, split this into two independent loops, and then heat them alternatingly, one at ~ 30 °C and the other at ~ 55 °C? Or just use two separate heat pumps? I mean, I have no idea whether any of that makes economic sense, but it really would be unfortunate to not use the potential of the combination of a heat pump and the low temperature required by under floor heating.
Ah, yeah, and beware of FUD. There is a lot of FUD around heat pumps, even ITT. While over-optimistic calculations are a thing and bad installs by inexperienced installers seem to be somewhat common, much of the horror stories you hear are told by people who obviously have no first-hand experience and are just parotting something they heard somewhere without any understanding of the respective context, but they still are absolutely certain that the only reason why some heating system doesn't perform as expected must be because of the heat pump that happens to be involved.
One pitfall in particular when comparing COP numbers you see people report is that hot water potentially has a huge effect on COP. If you are in a well-insulated house with under-floor heating and with many people who take a bath every day, you can easily end up with, say, 4000 kWh of hot water produced at ~ COP 2 (thus 2000 kWh electricity) and ~ 8000 kWh of heating produced at ~ COP 4 (thus also 2000 kWh electricity) with a total (S)COP of 3. Same house, fewer people, less bathing, and you might be much closer to (S)COP 4.
Also, if you can read German, the haustechnikdialog forum can be an invaluable resource as to experiences, DIY solutions, hacks, optimization, common planning mistakes and the like:
https://www.haustechnikdialog.de/Forum/