The easiest way to plot a route, using OS maps, is on https://www.walkhighlands.co.uk/maps/
(you can also get there from the home page https://www.walkhighlands.co.uk/ by selecting GPS on the menu).
There's a login link on the left of the GPS page, where you can register and login to access 1:25,000 OS maps. However, you can use 1:100,000 maps without login.
To start a route, drag and zoom then click on your starting point. You then click out the route you want to follow. Use the Your Route box on the left to name the route and there's a Download to GPX or KML option at the bottom of that box.
It's a simple as that - there is no option to upload GPX files or to save routes in WalkHighlands.