I've found that sometimes, if you call the SIN phone check-in they will assign seats for you (other times, they will insist that only ex-SIN flights are eligible) Alternatively, if you call the UK reservations line T-48hrs and ask them to assign seats, they might be willing to go into airport mode and assign your seats that way (Seat reservations in Y not allowed in Reservations Mode, but once the flight's handed over to the airport then some (/all?) agents can enter Airport Mode and assign seats even when flying Y.