Hi Si, we're living in our 8-year-old new-build eco-house in Wadebridge. We haven't used the finishes you are planning, mostly Doug Fir from Duchy and Red Cedar shingles from John Brash. But you might be interested in coming to meet, as I can talk you through some of our details and building structure decisions to get to £33 a year space heating [albeit will be more this year with crazy elec prices]. BTW, we have got Planning on our next project, a barn conversion [with new-build extension] in North Cornwall. I also need to source some charred or similar black timber for a friend's project nearby.
Looking at your architect's drawings...If you're going for slim-sight-line sliding doors, you may have little choice ref Frame in Aluminium. However, I'd recommend you look very carefully at the Whole Window u-values when you get door and window quotes [the Uw figure, not the Ug figure for the glazing alone]. The Uw figures for Aluminium frame windows that I saw recently were appalling due to the poor Uf rating of the frame, and I'm certain will result in condensation on the inside of the frames [not the glass itself which is ok]. With your modern thin-frame window designs it may be less of an issue? Generally lower, better Uw with the timber windows with Alu facing so beloved of architecture magazines. And with your large glazing areas, triple glazing will cut down significantly on the draughts across the floor in winter [reducing the temptation to turn the heating up higher to compensate], worth getting a quote for [the cost of Triple v Double seems to vary, can be very little]