Comments on this please

Windows 3g, doors, level thresholds, 0.7 or better

Walls stone, lime pointed, 200 eps, vb, service void? Metal battens once fixed in the middle, locked to top rail so walls effectively hang from roof/ceiling, wall vb joined to roof vb done nicely. Splayed reveals with thermal bridging mitigation to all four sides, eps beads poured down back of eps from roof. Eps sheets continue up gable walls

Concrete ground bearing sub floor, French drain all round, Floor 200 eps, flooring WPB ply, wet-room with fail safe drain, dpm/vb on top of eps turned up between first and second 100mm sheet of wall insulation, floor floats on under walls by 100mm

Roof rc ring beam, wood framed with ridge beam, warm roof design, slates, dry verges with cloakers, insulation joins wall insulation, extra between rafters over walls on the outer line.

Drains to existing cesspool,

ASHP, mvhr,

The ground floor is a floating plywood one on 200mm of eps and the ply ende 100mm into the wall insulation to help spread load of the walls, join everything up nicely.

How does the airtightness barrier connect at the floor/wall junction and at the wall/roof junction?

Also how is the insulation continuous at the wall/roof junction?

When you say metal battens, do you mean metal studs? I think you'd need to talk to the manufacturer if you wanted to use them without a base channel. Where are they in the build up?

You could probably support the base channel on a ply sheet on top of the underfloor EPS if you used the right grade EPS (maybe standard would be stiff enough, or maybe you'd need a denser grade around the edge)

That's pretty much what we did with our place,

Our floor needed digging down to add insulation, we poured new slab on top of the insulation. Stud wall spans from floor slab to ceiling.

Only comment is our granite/lime/rubble walls are slightly porous to rain, wind and mice. We left air cavity between stone and insulation to allow rain to drain and be dried by wind. Previous owners insulated in contact with the stone, which got soggy under the vb. The stone doesn't contribute thermally once the insulation is inside it.
Mice seem to prefer EPs to PIR, so using latter, and avoiding service void.

On similar projects nearby the thinking seems to be demolish the stone building, replace with bigger/better/warmer TF kit, then face the front with recovered stone.

Get a quote for filling all the space between and behind your studs with Icynene, it will be a LOT quicker then cutting the EPS, and will give good robust airtightness as well as being breathable. They can also do the calc to prove you don't need a VCL.

Also is the Concrete ground bearing sub floor new, if so consider putting EPS under the concrete with 100mm of EPC round the outside walls so so the concrete does not touch the outside walls, then build the studs on the concrete.