This tool allows you to search the Highways England database of roadside markerpost locations. You can download the raw data as an Excel spreadsheet from this location.
Use the map search to pinpoint a location and show markerposts within a 250m radius of that point. A maximum of 80 results will be shown within that radius.
Click or tap the map to add a marker indicating the point of interest. If you put the marker in the wrong place, refresh the page. I've spent three hours trying to code a way for the marker to be deleted and have come to the conclusion that my life is not long enough to learn the stupid way that OpenLayers handles click events. If I ever meet anyone who helped code the OpenLayers API I will strangle them with my bare hands.