I think you've unintentionally created a great example of how and why public discourse and lawmaking are so broken.

You invoke "safety" in the same manner that peddlers of all sorts of evil invoke terrorism or think of the children and then you cap it off with a straw man, as if there's serious money to be made with or without this mundane and niche law or comparable ones.

Bridge height postings more or less stand on their own merit and probably don't need laws to continue to exist. The fact that they are legislated at all is mostly a reflection of the fact that the state was the only entity positioned to deal with such an issue when they first became of enough value to be worth doing basically all the time.

I think you'll find that this law does not originally come from the state. It comes from local river authorities, perhaps under the Port of London Authority.