My main point earlier was that often the predicted times/heights from the curves don't agree with the reality on the day, so then the Rule of Twelfths is OK for practical purposes. You are right though that some areas are weird/extreme enough that it may be inadviseable. And the upper Severn Estuary is world-renowned for its wacky tidal behaviour!

And oddities such as Ramsay, the Solent, parts of the west coast of Scotland etc require a bit more local knowledge I understand