If the Brace Position procedure is part of the validated emergency SOPs, you would expect the CAA to ensure that what is documented on the safety card and given out in the safety briefing is actually capable of being achieved, with a practical demonstration using people of representative height and girth, like the aircraft evacuation test.
Richair, the RAF "backwards" bit seems to date from a "Good Idea At The Time", long ago. Boeing more recently did a much more sophisticated examination of survivability with the seats in either direction, and found that forward facing was better, because a significant number of serious injuries are actually caused by debris of all sorts (baggage, galley equipment, other unrestrained pax, etc) flying forward up the cabin in a sharp deceleration, which forward facing seats give much more protection from than rearward.