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Nice tip on the buffer vessel. Will ask.
We’re zoning by upstairs and downstairs, so will have thermostats upstairs/downstairs (I assume!). Reason being they recommended a split system of gas/ASHP due to heat loss calcs. To make sure we could get off gas 100% we agreed that 18 deg upstairs, 22 deg downstairs would be fine. Given we’ve never lived in a house where we’ve ever had a consistent heat of 18deg anywhere and don’t like hot bedrooms that works for us.
andos-
How big is your house?
I'd bet big that your heat loss calc is overcooked, everybody's is. Ours came to an alleged 9.5kW total loss at -2degC, but the actual measured loss is ~6kW at -2degC. I had to fight to get the originally-specced 10kW Vaillant bumped down to a 7kW.
Zoning doesn't really work in practice, the warm rooms will always be losing heat to the unheated cold rooms causing flow temperatures to increase and the heat pump to work harder. It's also always more efficient to have as much water volume circulating as possible; shutting off radiators and reducing volume causes more on/off cycling which kills efficiency. Much more efficient and comfortable to balance the whole system properly and constantly heat the entire house as a single zone.
ectoplasmosis
Sounds like your place is already much more insulated than ours!
Glad you're getting a Vaillant, they have very effective control logic, meaning even though your 12kW will probably be oversized (the vast majority of heat pump installs are), it doesn't matter so much as it won't egregiously cycle on/off unlike most other brands, and bigger heat pumps achieve higher efficiency when heat demand is high in deep winter.
Make sure you ask whether your installer is planning to fit a buffer vessel or not; they reduce efficiency by a fair whack. Best practice is no buffer, no zoning or thermostats, entire house run as a single zone using weather compensation.