Using these numbers, 77 kWh (factoring DHW and efficiency) over 24 hours at Δ10°K* gives a rough heat loss coefficient of around 320 W/K for the property. Scale this up to a design temperature of -3°C (Δ20°) comes to 6.4 kW heat loss for the property. Even with defrosts, a 18 kW unit is way oversized.
[*active heating is often not necessary when outdoor temperature is above ~17° as there’s sufficient heat coming from appliances, humans, sunshine, etc. Can see this on the first chart you posted. So calculate Δ from that temperature rather than actual indoor temperature. See heating degree days for more info on the concept.]
You can get more accurate figures by plotting daily heat against average Δ and fit a line to it. Compare both gas and heat pump data, and you should get similar results (adjusting for any improvements to the fabric).
4.4 kW, as mentioned in post #12.
As Dave notes above, the stable flow temperature is dictated by how much heat the radiators can emit. If your radiators are too small or too few or restricted in some way or there’s a bypass, then they won’t be able to shed the heat coming from the heat pump, and flow temperature rises. This could explain why consumption and COP is worse at 50° than at 55° - the system is cycling more, and being less efficient.
It would help to have a chart of flow and return temperatures throughout the day, as this will help us see the behaviour of the heat pump. The flow rate is also useful to know, as this can be used to calculate actual heating power.
Daikins are often accompanied by an automatic bypass valve to protect the system if the primary flow is restricted. This is unnecessary if you have a low loss header, so look to see if you have one, and make sure it’s not passing hot water.
There will be a 3-way valve that switches between central heating and domestic hot water. If this is broken, this can allow some of the hot water to bypass directly to the return, raising the flow temperature. This is precisely what is happening on the topic Mitsu Zubadan Flow Temperature Issue - check that the DHW side isn’t hot when not active.
The low loss header can impact efficiency a little, but not enough to cause the issues you’re seeing. Ideally, the secondary pump is matching the speed of the primary, and the temperatures on both sides of the LLH are equal. There is a process to balance these, but it’s quite involved. The topic linked above covers this too.