You may use this map screen to plot routes, or to upload GPX routes. You can download created routes as GPX files.
A selection of map types (source) are available; to use OS Maps click the Layer icon (top right) and select Ordnance Survey. 

The map surface comprises the following four control groups:

• Top Left
  • Elevation Profile
  • Map Tools
• Top Right
  • Help
  • Settings
  • Map Type/Source Selector
• Bottom Left
  • OS Grid Selector
  • Location Search
• Bottom Right
  • Toggle "snap to path" on/off
  • Start plotting
  • Add a marker
  • Edit route and-or markers
  • Delete a route
  • Reduced plotted points (simplify)
  • Upload GPX
  • Download GPX