From my experience and that of others I know, many installers are put off by knowledgeable customers. It does at least seem that your BG energy surveyor knew what he was doing, or at least trying to do.
7 years ago I was quoted for an 8kW system. I have made a few improvements since then and was happy to get a 6kW one.
I suggest that as part of your preparation works you could upgrade your known problem radiators with massively oversized ones. Gradually reduce your boiler temperature until it is running at heat pump optimum temperatures. You are then ready for a simple slot in replacement. My own boiler ran at 40°C except in really cold winter spells, when I raised the temperature to 45°C. I was aiming to get lower but when the BUS came along I thought I should take advantage while I had the chance and jumped ship.