Do you want to hear my Cellotex and multifoil recipe? easily achieves airtighness, eliminates summer overheating, just scrapes thro Regs according to the weedy Thinsulex accreditation (which makes any kind of multifoil OK with Bldg Inspectors) but will in fact give superlative performance.

Roofline can't be raised so the battens must sit direct on the rafters so ROOFING FELT MUST BE ALLOWED TO DROOP INTO THE RAFTER SPACE SO TOP OF cELLOTEX MUST BE 25 BELOW TOP OF RAFTERS (sorry, caps lock).
As rafters are so skinny, add minimum 45-50 down-slope battens to deepen the underside of all rafters, 70-75 better. Fit 60 or 80 Cellotex very neatly between the rafters, no gaps, edges bedded or 'pointed' with foam. Cellotex top 25 (diagonally) below rafter top, underside of Cellotex minimum 35 (diagonally) above underside of the deepened rafters.
Spotwelded-type multifoil across the deepened rafters' underside - not the perforated stitched-type multifoil. At present, as far as I know, that means Euroform Xfoil, as manuf'd by my mate Paul Mitton (I'd like to know if others start making non-perforated multifoil). Only the non-perforated type can act as the airtight barrier.
35h x 50w counterbattens across the rafters' underside. Note that the multifoil (nom 30 thk) has 35 up into the rafters' depth and 35 down into the counterbattens' depth - that's the only way to minimise contact - the usually recommended 25h battens are near useless.
Plasterboard soffit. Gives wiring or even pipe space above the plasterboard. Recessed spots - must be cool-running heat-downward (not dichroic) type, or LEDs, can be fitted _ i have a detail/spec to prop the multifoil up locally.
Airtightness - I might consider publishing details of how to join multifoil to multifoil (mechanical clamp only - don't think of using sticky tape), multifoil to Velux frames (requires non-standard trimmed rafter layout), multifoil to wet plastered gable wall, and how to wangle the airtight membrane past the purlins.
As well as airtightness and elimination of solar overheating, this spec should give a whole-heating-season heat loss performance equivalent to about 210mm of Cellotex - but be warned that that's not as good as it sounds - 210mm of Cellotex (or any conventional insulation) will actually lose about 2.5x as much heat as you'd expect from blindly applying its published steady-state k-value.

......."210mm of Cellotex (or any conventional insulation) will actually lose about 2.5x as much heat as you'd expect from blindly applying its published steady-state k-value. "

could you explain that please?

Yup, Id pass that tom....