The Department of the Environment, Food and Rural Affairs (DEFRA) have a free on-line digital mapping tool called MAGIC Map:
https://magic.defra.gov.uk/magicmap.aspx and it comes with an easy to use online help file.
Amongst the various mapping layers, the familiar OS 1:50,000 and 1:25,000 colour mapping can be selected and areas up to A3 size printed at scale to PDF. There is also an aerial imaging layer and these two layers both have transparency sliders which greatly facilitates swapping between the map and imaging when route planning. Other selectable layers include CRoW access areas, registered common land, national trails, English coast path route and much more.
You can use the line drawing tool to plot a route on the map. It is easy to edit the line of the route and swap over to the aerial imaging layer to fine tune it if you wish. When you have finished can then export it as ZIP file. As yet, the program does not currently support export to GPX and will not create a Google Earth KML or KMZ file either.
However, the ZIP file contains a number of files and amongst them you need to find the one called “magic_lines.shp” which is a standard ESRI shape file containing the georeferenced trackpoints and this file can be readily converted into a GPX or a KML file.
You can either do this online using one of the free converters (Google: SHP to GPX converter) or download a free app. Either use the DNRGPS app from
https://www.dnr.state.mn.us/mis/gis/DNRGPS/DNRGPS.html which will do this on your computer and works with Garmin devices or GPS Babel
https://www.gpsbabel.org/can to do the conversion. You will need to check that the converted GPX is read by your navigation app or device before rushing out to the hills to follow it!
You can also import tracks and routes as GPX, CSV or as a ZIP file which contains a GPX or CSV and view them in MAGIC. You cannot at present upload KML or KMZ files. You can choose colours for the track, trackpoints and waypoints before you load the file and these will be displayed on the selected mapping or imaging, but there does not seem to be any editing functionality at present.