I'm having a hard time understanding your justification. Ebbsfleet is 25 miles ESE of London (Ashford 50+ miles), with no local (TfL) connections. I'm also curious to know the basis for your claim that "the vast majority" of Eurostar customers are from north of London. I hear lots of people live in Surrey, not to mention London proper.
My point is that London is the south east, by bringing the erostar round to north London it is easer for the whole country to use. There is probably an argument that they should have a station in South London but the Erostar is there to cater to the whole country, not just london.
