indeed the 4pm departures from LHR with the 1/2PM arrival in BKK/SGN or 5pm arrival in Tokyo works perfect for me. Often times I want to fly out Friday night and arrive by mid afternoon/early evening Saturday to maximise leave plans etc.

Also depends what matters for you, for me personally I’m always rushing to these flights after work so won’t have time to always eat beforehand etc so I prefer airlines that feed me well in-flight…for the same reason I’ve avoided JAL longhaul in Y some times due to their skimp-ish offerings (no more Finnair long haul in Y either after a nearly starving experience 14h ICN-HEL) - you’re most guaranteed to be well-fed in QR even in Y with 3 choices for each meal service (very rare these days).