When teaching PFLs I have found it one of the more difficult exercises for a student to master. It doese require skill, judgement and regular practice to pull off a successful landing into a field in the event of an engine failure.
The last 500ft are demanding and stressful for the student and it is only by allowing him/her to descend below 500ft can you build up their skill.judgement and confidence.
In my view if the CAA amend Rule 5 to include the 500ft heght limitation there should be an exemption for engine failure training.