After months of messing around in the editor I am now going to try and build a real route! Eastbourne to Ore is the next extension along the east coastway. Should be a fun project!
You can´t edit existing routes, so no, you can´t join it to the existing BML, sorry. But I thought that Matt said somewhere, that they´re want to make that possible, so maybe one day.
It might be worth discussing with the creator of the London thread, as it looks like SEHS and Goblin Line are present in his own work