Could it be that you are in UK now and that MPO has not enough time to send you the ticket?

If your ticket requires an advance purchase, it has to be issued 'now'. Even though you may be leaving ex-HKG, what are they supposed to be doing with that ticket after it is issued.

If it is a special fare, then it cannot be issued on day of departure, nor can you pick it up from another country.

That is probably why MPO is saying that it will be an e-ticket. You didn't get more specifics so I can only guess what transpired.