I'm not sure exactly what's going on, even if you provide a US address in the middle of the country it will only ever give you directions for the European side. If you replace Heathrow with a different continent, like Africa or Asia, it correctly says it can't find a route. My best guess is that the algorithm sees there is a water crossing, but then finds the ferry from France to England and says "ah ok all good". Maybe there is a special case to handle Europe to UK that is causing the problem? If you put in JFK to Dublin it will find two ferries, France to England and England to Ireland, but still nothing on the North American side.
Edit: No nothing to do with the ferries, I think Europe just has some special status. For example, try Brazil -> Morocco vs. Brazil -> Spain.
I'm not sure exactly what's going on, even if you provide a US address in the middle of the country it will only ever give you directions for the European side. If you replace Heathrow with a different continent, like Africa or Asia, it correctly says it can't find a route. My best guess is that the algorithm sees there is a water crossing, but then finds the ferry from France to England and says "ah ok all good". Maybe there is a special case to handle Europe to UK that is causing the problem? If you put in JFK to Dublin it will find two ferries, France to England and England to Ireland, but still nothing on the North American side.
Edit: No nothing to do with the ferries, I think Europe just has some special status. For example, try Brazil -> Morocco vs. Brazil -> Spain.