Pace, I know you know this, but FL measures atmospheric pressure (albeit in units of feet), so it is exactly what the regs should specify.
It is the altitude which is less accurate, though as Backpacker said, altitude may be more practical if the TA is high.
Don't you have that backwards?
If I'm standing on the shoreline, at the time exactly halfway between high tide and low tide, then I'm at mean sea level. If the pressure at that point (at MSL) is 1000hpa and I set my altimeter subscale to 1000hpa then my altimeter would accurately read 0ft.
If I set it to flight levels (1013hpa) then it would read 390ft (13*30) which is FL003.9 (Not that you'd ever replort FL003.9 to anyone!). While using a standard scale is convenient for aircraft seperation, it is less accurate than altitude.
I'm standing at MSL (shore line half way betwen low and high tide) and altitude is reading 0, and FL is reading 390ft. So altitude is more accurate. In terms of the effect on the body using 0ft in your calculations must be more accurate than 390ft at that point.
I think that is the point that Pace was trying to make.
But of course in the UK an aircraft at that level will have their altimeter set to 1013 and the differences in effect on the body are likely to be small. The change is risk of hypoxia is probably considered to be far smaller than the risk of someone having a miss-set altimeter due to changing it to check the pressure altitude and back to FL.
dp