I just arrived back from SYD yesterday (BA 16 all the way), and agree waiting at least three (if not four, if possible) hours after takeoff from SIN. I fell asleep immediately after dinner, and then woke up with hours to spare.

You shouldn't have any problems getting some time in the lounge at SIN; keep in mind security is done at the gate.

Lucky you flying F! I was in WT+, so it was bearable but not particularly enjoyable for a 24 hour trip. Perhaps that's why I slept all day yesterday once arriving home in LON. I pushed myself to still sleep through the night, so appear to have gotten over the jetlag today.