As far as I see there is no accessible highway connection between France and Britain. No Street → no routing.
Using trains for car routing is not supported by routing apps. Of course not.
There is only a theoretical connection using an proposed feature shuttle_train which seems also not supported by routing apps.
This feature should be similar to using ferry boats. Maybe this hint can be useful for your programmer?!
Thanks - I’ll try that (and your advice comes just at the right time, as Scout are promising “a really major update, hopefully in the first quarter of 2016”).