Update, July 2021. OS have updated their OS Maps so that downloaded files now contain trackpoints but without elevation or date/time information. These can now can be uploaded to GPS devices or OSM applications without conversion. The files also contain a waypoint for every trackpoint with the same position information which is unecessary and causes problems in some Garmin devices.
Another useful characteristic of GPS Visualizer is its ability to combine multiple tracks into single trackfile (of 5Mb or less) which can then be uploaded to OS Maps. Dozens of tracks can be uploaded at the same time but only one track can be viewed at once. Non-subscribers can view a track on a Standard OS map but subscribers can then view them on 1:50k and 1:25k maps and download the .gpx file.
An alternative method of displaying interactive OSM maps is to use Leaflet (https://leafletjs.com/) a JavaScript library for interactive maps and which is what Ramblers use to display route information on OSM. An example of this showing walk start points can be found on our website.
Use of Leaflet requires a knowledge of coding in Java but this can be avoided by using an option in GPS Visualiser - (a few dozen) individually coloured tracks can be shown on an OSM map, an example of which is shown here.