Now a person may be passing Basingstoke on his way either to Southampton or Plymouth, both of which are ports of embarcation for various foreign countries.
I attached importance to that clue.

Something about the tone of Hilda's letter made me realise that she intended to put the sea between us.

In concluding so much, I felt sure I was not mistaken.

Hilda had too big and too cosmopolitan a mind to speak of being "irrevocably far from London," if she were only going to some town in England, or even to Normandy, or the Channel Islands.

"Irrevocably far" pointed rather to a destination outside Europe altogether--to India, Africa, America: not to Jersey, Dieppe, or Saint-Malo.
Was it Southampton or Plymouth to which she was first bound ?--that was the next question.