Is approx my standard build-up, based on WUFI comparative studies some years ago, using Brussels weather file that's prob good for southernish England. It worked considerably better, vapour/interstitial condensation-wise, than using a vapour barrier, even an expensive 'intelligent' one. It's all about unimpeded re-drying ability.

Building Inspectors have accepted it without question, but there seems to be a new breed of outsourced BldgRegs plan-checking service, which don't do any common-sense decisions, just everything by the book. After presenting the resultant charts that I kept, I've been asked for fresh WUFI calcs, must be presented exactly like a 'qualified environmental engineer' would do it incl the chapter-and-verse listing of all relevant BRegs clauses which 'independent consultants' do to bulk up their 1-page report into something they can charge high-£100s for. In other words, this is a non-standard construction that you won't find any authoritative certification for - so, excellent as it is, it has to be proved afresh.

Differences from my recipe -
- definitely no 'vented air gap' in the stud area. The 100 stud void blown full of Warmcel - could be other breatheable insulation, like quilt.
- 11 OSB3 gapfiling glued and screwed/power nailed incl to noggins at unsupported edges. That's fairly airtight, no need for taped joints.
- 150 EPS glued to the OSB. PIR is not breatheable, will 'trap' as you say, regardless of 'not trapping' by VCL (or no VCL).
- the rest - counterbattening, vented cavity, cladding - is fine.

Other comments -
- the inboard pbd & skim doesn't need to be airtight or vapour tight, can be punctured at will by electrician etc.
- there's no virtue in vapour 'retardation' - best it breathes fully. The OSB3 has slight vapour resistance, as does the EPS, nothing to make a difference either way.
- OSB3, blown-in Warmcel are both fairly airtight, together adequately airtight, in a robust way not too sensitive to accidental/later puncturing.

Thanks Tom,
The 'vented air gaps' exist simply as a result of building a 400 centre stud wall but will function nicely as a utility void - I decided it would also be a good idea to vent them - on reflection I doubt it's doable and I guess not needed.
I could insulate between studs but the 120mm PIR reaches my wall target without need for addition (and creates the right wall thickness to sit the external cladding just past the edge of the brick plinth below)
Does the use of PIR seem sensible to you? I could also offer additional insulation between studs as an option.
I understand that EPS breathes a little, unlike PIR and would like to try it on the next project as you've described - but could the carefully taped, unbroken layer of PIR over OSB3 cause any problems that I'm not aware of?
If under certain conditions a dew point existed inside the PIR thickness (which is closed cell) would it matter?

wondering why the studs frame is 125mm and why 400mm centres? Is it also load bearing in addition to the structural oak frame?

Yeah, but it's OK - if 125 studs filled with blown-in Warmcel, then 125 EPS - as long as it adds up to 250 (magic number)

As I said elsewhere I used Icynene (an open cell foam). Slightly different buildup as it was a light steel frame, 150mm PIR outboard of the frame then the Icynene blown in. The WUFI I had done showed the dew point was in the middle of the PIR so no bother with it not breathing. I hit the magic 250mm

I'd put smaller studs and more outboard insulation, plus some in the frame.

Just to note, getting PB behind the Oak is a PITA.