Hi @ash, and a warm welcome to the forum.
You are asking the right questions, and hopefully you will get plenty of learned answers here. I’d just make a couple of general points:
- No matter what your heat loss actually is, remember that it’s what your installer estimates that’s important if he will be applying for the all-important BUS grant on your behalf, and that he has an incentive to oversize your heat pump. (Two incentives actually - he makes more profit from selling you a bigger HP, and you are less likely to complain that your house isn’t warm enough.) So be prepared for a debate with him if you think his estimate is much too high.
- There is a (minor) downside to choosing a HP that just matches your maximum demand - you’ll have less scope for roomstat setback at low outside temperatures, as discussed in House Thermal Inertia and Roomstat Setback (some cautionary notes). (Personally, my economics favour a night time setback, and I’m glad I chose a HP with a couple of kW spare for house reheating in the morning.)
- It may pay you to review several HP manufacturers/models, and then choose an installer familiar with the brand. If you choose your installer first - especially if it’s one of the nationwide ones - you’ll end up with his standard brand, come what may.
- As you will soon see on this forum, there is no perfect heat pump (they all have good points and bad points). But in general, I suspect that you get what you pay for…
Keep us informed of your progress!