How can a loft be most energy-efficient, with the 'equivalent 10mm air gap' removing the heat?

At eaves level that gap can mean a big draught.

OK, how about this idea - have the eaves gap but keep it contained, and only ventilating the ends of the rafters, joists, wall plates etc. Insulate the rafters but hold the insulation in with some sort of mesh so the bottoms of the rafters are exposed to the loft, which is heated. Would this get through the regs?

You need a certain amount of airflow through a loft space to carry moisture-laden air away, otherwise you get condensation build-up, which can lead to damp timbers etc. As long as the loft insulation is installed between the through flow of air in the roof void and the house, this is accepted as being thermally efficient whilst preventing moisture accumulation.

as above says not only energy efficient it is building code, without you end up with moisture then possibly mold., the only time you do not need a vent is when it is solid insulation wich after r-50 becomes money ill spent ( you will never recover cost)

the 10mm air gap is for air circulation in your loft space which is actually the area of what is known as a cold roof. you should have 300mm of insulation between the ceiling joist's, which prevents heat loss into your roof.
if you had a what is known as a warm roof the insulation would be between the rafters and the air flow would be above the insulation.
hope this helps..

There are lots of considerations in the loft being energy efficient. What is your source of heating and are there registers on the floor?

I inspect buildings to show their energy use and it is amazing to see a reasonably insulated room except the heat registers are on the ceiling. People complain of cold feet because heat rises. Go to http://www.thermoguy.com/globalwarming-h... to see the winter function of the building regarding heat loss.

Is the loft very hot in the summer? UV and the same solar radiation that burns our skin is causing buildings to generate extreme heat. The bottom line is the building isn't designed for the generated heat so it transfers inside where you react to the symptoms.

Go to http://www.thermoguy.com/globalwarming-h... and see information that wasn't available before. There are avenues for you to quit generating the heat and quit having the problem.

You need to keep the air gap, so air can vent the loft space above the insulation. 270mm insulation is the current recommendation for the loft. above the insulation does not mater.

No that doesnt sound very energy efficient.

The energy efficiency is dependent on the thickness and type of material used to insulate the loft (the thicker, the better). The 10mm ventilation makes little or no difference, since the flow of air is minimal and above (ie outside) the insulating layer.