you used to be able to upgrade from economy using miles for 45k LHR-SIN....but alas, they now don't have the 747 flying from LHR so they doubled the miles needed to 90k for one way upgrade LHR-SIN.
You used to be able to book it online (providing you had booked full fare economy) but the new website is terrible and I doubt it has the intelligence to do this anymore.