Sennokot is okay, physllium husks okay, Fybogel natural only not flavoured, but not dulcolax. It used to be allowed but several people have had severe pain and collapsed after using this on a VLCD so it is now not recommended. Prevention is better than cure so Fibre 89 should be used from the outset.
Once constipation is over then introduce the Fibre 89 daily to prevent reccurrence.