Thanks for the replies!
In answer to @ScottishJohn, I'm hoping to use some builders to get the construction done quickly, but I'll be managing the project. It's all happening in one go. Apart from the potential for pausing construction, what are your thoughts on the polyblock vs woodcrete decision?
Regarding cavity wall construction, that really is another topic, but I've been working for the Cheese project in Bristol doing thermal imaging surveys of lots of different houses, and this is a very good way of getting to understand the real thermal performance of different building methods. When you measure cavity wall buildings you discover that they really don't perform well at all. Rather than go into all the details, here is a link which discusses it if you are interested: https://www.designingbuildings.co.uk/wiki/Defects_in_dot_and_dab. It references dot and dab plaster finishing inside which essentially shows up the difficulty of preventing cold air getting throughout the structure, but even without this, external air usually finds its way into the ceiling to floor voids, under the skirting etc. The problems are all hidden unless you actually measure things with a thermal camera and air pressure fan, and they are very difficult to rectify. It's a bit of a scandal really.
Anyway, what I've learnt has convinced me that the way to go is solid wall with external insulation, or ICF.
Thanks for the offer @Willbish, I'd love to see what you've built! Did you use anyone experienced with the ICF system? And which system did you use?
Durisol was recommended to me and I had a look at it. To get the sort of insulation factor I'd like, the walls come out at 400mm thick including internal and external finish.
Thats some very extended list of info @AnonymousBosch! To summarise, get your design right before starting and use then right workmen?