Swimming pools are covered by the Fencing of swimming pools Act 1987:178 Which is administered by the Department of Building and Housing.
There are exemptions to the definition of a swimming pool which include: Any excavation, structure or product which is less than 400mm deep. And
also any excavation,structure or product that has not been modified for use or intended to be used for swimming, wading,paddling or bathing.
Since this is an Act of Parliament it is not legal for a Local Authority (including one with John Banks as Mayor) to change the rules as they see fit.
The words mean what the words say, and they say:
Any swimming pool less than 400mm deep does not need to be fenced as it is not a swimming pool.
A gold fish pond that has not been modified for use or intended to be used for swimming,wading,paddling or bathing does not need to be fenced as it is not a swimming pool.
So build your pond deep enough for you fish to survive the frosts down here and breed and keep some realy good quality fish.
The fact that a child can drown in a goldfish pond or in water less than 400mm deep is not relevent to what the law requires. Any interpretation other than this by a Local Authority in my view is ultra vires
Have a good day.