I flew LHR-SIN (744) on a spacebed seat 22C, bulkhead main deck. It seems it was just okay it terms of the space, but of course exit row 17 upperdeck was the best. Anyhow, you can reserve bulkhead seats any time, except exit row upperdeck than only can be assigned in advance for their Solitaire, PPS, or Gold KrisFlyer. I was able to secure bulkhead seats all the time in 777 or A340 without any status on SQ.