I'm using OS OpenData in geopackage format exported using qgis. I've managed to use and epsg:4326 projection, however, i found some files of yours on the wiki of borough boundaries and my files are still off from your files. If you don't mind, could you possibly explain the process on the particular transform you are using?

Hi there, sorry for the delayed reply. What I do is with the boundaries, I export them as points to ensure high accuracy. The reason for the offset I assume is because when boundaries have been drawn previously, they have been drawn roughly based on line geometry not the specific points. I hope this gives you better understanding.