As Andrew stated, there is no, nor (do I believe) has there ever been, a 'legal' height under which you did not need handrails. IIRC the 2m (or was it 6ft?) figure came from a specific regulation related to the construction industry. Because there was nothing else to go on, our industry adopted this as normal practice, but it was never even an 'official' guideline.
You need to carry out a Risk Assessment, which to a large extent will consider the Working At Height Regulations. A fall from any height has the potential to cause harm. You can step backwards off a 6" high deck and fall and bash your head, resulting in serious injury or worse. Standing next to an open trap door is Working at Height as there is potential to fall. Control Measures to allow an unprotected edge might include careful rehearsal, blocking actors to keep a minimum distance from the edge and making the edge clearly visible. None of these will be as good as having a sturdy handrail of some sort, so give serious consideration to that as an option.