I hate a misplaced apostrophe as much as anyone. And I really hate unnecessary capital letters.
But, as any editor will tell you, the real reason for describing a piece of writing as bad is nothing to do with breaking the ‘rules’. It’s to do with communication. Ultimately, crimes against English happen when something about the writing prevents the message from getting across.
And this particular message – repeated at several places along the Thames Path – was very clear. And undoubtedly, for many of its readers, very welcome. I don’t think they would have cared about that apostrophe at all.