Excellent

Spent on/off the last three days trying to get a WordPress website HSTS eligible which runs over the www sub domain and after constant messing about with umpteen configuration and https/www rewrites, nothing would work to pass HSTS preload checks for subdomains.

Installed this plugin (handles all wildcard http >> https redirection), set the Strict-Transport-Security header in .htaccess and worked perfectly passing HSTS preload check for browser preload list submission.

Life saver of a plugin!