I tested with London to Paris — if it's easy with your routing system, you should add the Channel Tunnel as a fake road. I'm not sure why it chose a ferry so far off the shortest route.
The site could probably charge extra for better quality results.
Perhaps they could split it into a free version using open street data, and a paid version with actual transit routes and other relevant data (tunnels could be marked as 0% sun coverage if they are able to find a data source for it)
The routing takes the Portsmouth-Cherbourg ferry, which is not the way any bus would go. They take either the Channel Tunnel, or one of the shortest ferries like Dover-Calais.