This doesn't have OS maps, but it does have useful 'OSM Cycle' maps covering the world. You can use the Route Planner to plot cycle routes that follow roads, or click them manually. The Route Planner has the useful Google stickman, which you can drag onto the map to view junctions and see what the road is like.
1) Log in at https://ridewithgps.com/ (you will need to create an account first via the Log in link)
2) Select 'Route Planner' from the top menu. The map layer defaults to 'Map' in the drop-down at top-right of the map panel. Drop down this menu to select the 'OSM Cycle' mapping.
3) On the Overview panel on the right, scroll down and set options:
4) To start routing, at the top of the right panel enter a postcode or placename in the Enter a Location box and click Go. This will put a placemarker on the map - say Yes to accept it, then drag it to the exact starting point.
5) Go back to the Enter a Location box and put in your destination. Repeat the steps to accept it and it will automatically plot a route.
6) Adjust route: Hover over the route and a blue dot will appear. Click and drag to adjust the route. This will add Control Points along the desired route. Repeat as needed along the route until it's correct.
7) If the Follow Roads route doesn't allow you to go where you want, ensure you have Control Points either side of the route section, switch to 'Draw Lines' in the right panel, then adjust the route between the two Control Points'.
8) To add the return leg: Ensure there is a Control Point just before the end point. Drag the end point to near the original start point. Adjust the return leg as needed.
9) Save route: Click Save in the left panel. Fill in the details, including making it public if you want to share with others. Click 'Save as New'.
10) A new box appears: Click 'View your route here'.
11) To export to GPX: Select the More button in the left panel; Select 'Export as File'; Select 'GPX Track' and save the file to disk. It can then be uploaded to a mobile.
Click the bicycle button (top left) to get the full menu up:
The Routes option lets to View, Edit or Delete your saved routes.
The Rides option lets you view any uploaded rides.
The Upload option lets you upload GPX files to create Rides.
Convert a Ride to a Route:
1) Select Rides (https://ridewithgps.com/rides) to view your list of uploaded rides.
2) Click the ride name to bring up the ride map.
3) In the Overview tab in the right panel, scroll down and select 'Copy to My Routes'
4) Give it a route name and select Private or Public, then select the 'Copy to My Routes' button.
5) You then get the options to Close, Edit or View this as a Route.
Elevation Profile:
This should display below the Map panel (works in Chrome, but doesn't show in Firefox). You can move the cursor along the elevation profile to see the altitude at any point on the map.
When viewing a Ride there's an option in the right panel to 'Get profile image'. This opens in a new window. Right-click the image and select 'Save image as...' to save it to your disk.
Get Map Image:
When viewing a Ride there's an option in the right panel to 'Get Map Image' providing an image of the entire route. This opens in a new window. Right-click the image and select 'Save image as...' to save it to your disk.
Reverse route:
When editing a Route, select the Edit drop-down in the left panel and select Reverse Route. This will allow you to rebuild the route in reverse order, using existing Control Points and allowing for one-way streets. You may need to make some adjustments after the process completes.