Regardless of the licence status, if the airfield owner has a displaced threshold then you are required to abide by that. If you feel that there is no need to have the threshold displaced then speak with the aerodrome owner / licence holder. Very often thresholds are displaced for mobile obstacles outside or inside the boundary. 999 times out of 1000 the mobile obstacle may not be there. Other reasons can be noise abatement.
Having established that your landing distance including any safety factors permit a safe landing on the declared runway, why would there be a need to cross the threshold at less than the standard 50ft.
If on the other hand, you can make the figures match and you need to be less than 50ft at the threshold then ask yourself what those with 20/20 hindsight would think of this.
An approach to touchdown at a point before the displaced threshold would not be operating in accordance with normal aviation practice and would not be exempt from the low flying rules. Furthremore the insurance company and most likely the CAA would consider such an operation reckless should anything happen.
I honestly can't ever see a reason for intentionally landing before the threshold of a runway.
On the training aspect, a landing before the threshold would be a fail item. Crossing the threshold at an inappropriate height is at best a discussion point.
Look at the figures in the AFM. They are all based on crossing the threshold at 50ft at the correct speed in the correct configuration. Everyone needs to remember that a flat approach crossing the threshold at 10ft can often require a longer distance than a glide crossing the threshold at 50ft and the longest distance of all is caused by pushing the nose down late in the approach and diving at the threshold not to mention the unstable approach aspect.
Remember that most of the landing distance as published in the AFM of light aircraft is airborne distance!!
Regards,
DFC