Which OS data are you using as your source?

MHWS and MLWS obviously aren't coincident here. They're skerries, not cliffs, and 3 different aerial imageries (Bing, Esri, and Mapbox) show the rocks covered different amounts at different tide heights. Comparing imagery to MHWS and MLWS on the Monach Islands, current Mapbox imagery looks close to low tide, while Bing and Esri are somewhere between high and low tide. For these skerries, the tide line on Mapbox quite closely matches the Boundary-Line high water polygons (and admin polygons/low water, and OpenData StreetView MLWS). Therefore, the Boundary-Line data for these skerries is MLWS, not MHWS, and so is wrong. No, MH/LWS can't be determined from aerial imagery alone, but it is still useful for determining if an area of OS data is at all correct, especially when multiple imagery sources show different tide heights.

The latest OpenData StreetView is from April 2016, but no MHWS line is shown on these skerries at all on the newer, less detailed OpenMap Local (April 2024), presumably because they're too small. I don't know if the coastline here was resurveyed between 2016 and now.

I removed the coastline tagging because the pre-existing geometries were essentially outlines of the skerries at some mid to low tide point (they were tagged with Landsat as the source), and therefore seemed to me to be representing the skerries, i.e. the natural=bare_rock, rather than the tiny natural=coastline rocks I chose not to add.