Teatime. I managed to successfully to use a 2-4-1 Avios for two CW seats between LHR and Sydney earlier this year on the dates I wanted but it needed some organization i.e. booking the outbound at T-355 days and being on-line at 01.00 UK time, then calling the BA Exec number on T-355 for the return flight. So it is possible as two CW seats are released each day.
You get about 90 minutes in Singapore which is time to reach the BA lounge for a shower and couple of beers before reboarding.