Replying to myself to expand what I mean - if the roofing has been done correctly, there is air gap maintained at the eaves under the 'felt' to blow into the attic. That's still there, it's just now channelled through the gap between the rafters. In a 'between and below' scheme, that air gap gets narrowed to the 50mm or so left above the 'between' layer to allow that air to flow. In a 'below' only scheme the air flow ventilates the full depth of the rafter as well as the cold face of the insulation.Completely under leaves the whole depth of the rafter just as ventilated as it is with insulation over the ceiling joists.
Thinking back on what I did in my own attic, I cross battened before insulating below to make sure the air would have cross flow to avoid trapped air around jack rafters etc. Made it even more internal space inefficient (which is obviously the driver of 'between and below') but its only intended as a storage space.
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