Mr Punter

Nice job @DIYMichael. I really like the timber internal linings / reveals / cills for the door and windows.

No I didn't mention EWI. You need to fit counterbattens on top of the rafters and fix through the externally fitted insulation boards, hence long helical fixings needed. No need to fill between the rafters, although you could use a small amount subject to satisfactory condensation risk analysis.

If you have external insulation you will need to work out how to fix battens / counterbattens. You will need a lot of very long fixings. I think the helical type work well. Expect to spend about £7/m2 in fixings for counterbattens.

No, not viable. They are perforated clay and have poor insulation values. Warranty providers like a drained and vented cavity, so a single skin will not work. The blocks shatter if you drill them. You will struggle with plumbing and electrics. Airtightness is hard to achieve. More expensive than normal blocks. Just move on from these and save yourself a headache.

When the cavity is fairly wide the risk of voids is almost eliminated with blown eps beads. Probably the safest choice. Also if they miss, say an entire elevation it can be filled later.

Sometimes I have had the sockets on a flex under the units. Quite a few appliances have moulded plugs and it may be that any warranty can be voided if you cut them off. It is not simple to anticipate what is needed. I have had induction hobs needing hard wiring on a separate circuit where others are plug in 13A. I like under cupboard lights to be switched from an extra gang on the main light switch, which can also be a challenge.

Leica are a very good brand but IMO too expensive. The Chinese stuff is so cheap it is hard to say no.

You don't need to be qualified to build a house in the UK. ICF is fairly easy DIY. It would be more impressive if you built in brick and block. No requirement for Gas Safe if it is just for you but would be my preferred option.

The Dritherm 32 slabs are easy for the bricklayer to fit, easy for wall ties, decent u-value, don't make a mess if the wall gets drilled and easy to clean up mortar droppings. EPS beads are good too but you said you are not keen on them.

Proper rough job. It would still be rough without the rigid insulation, which elevates it to farcically bad.

Gas Safe plumber would be my answer.

Bear in mind that lots of EWI has been a disaster recently. Lots of mould, damp and ventilation issues, so this needs to be done meticulously, especially mixing EWI and IWI.

Try it on a small section.

Yes. Look up "Leaky Condo" and see the consequences of omitting a drained vented cavity on timber frame.

Dritherm 32 cavity slabs.