It's a bit difficult to judge what's actually right and wrong here. Establishing MHWS and MLWS can't be done from a simple aerial photo of course; it's a complex process. They can be coincident, and often are: vertical cliffs, quaysides, negligible tidal heights... Regarding OS data for the foreshore limits, I would suggest that the most recent data is the best we are ever going to get into OSM. Have you checked the survey dates on the various sources? Although in this case the actual line is unlikely to change very much over time, survey techniques do improve, leading to the later data being a better approximation of reality than earlier surveys.
Regarding the two larger outcrops:  you actively removed the coastline tagging, that's different to failing to add. OS also has criteria for islets in Boundary-Line, including a minimum area of 0.4 ha (at high tide). 