Ms G50 and I did LHY - HEL - BKK - SYD return in J last January on Finnair ticket last January. LHR - HEL was BA, HEL - BKK was AY, BKK - SYD was BA. On the return SYD - BKK was QF... the rest the same.
BA CE was ok and we booked an earlier flight to do the sauna at HEL.... ^
AY to BKK was good (flat beds) but tight.... check out the AY forum for more.
Watch for bad connections because the reps for AY at BKK and SYD can be a bit lackadaisical.
The good thing about this itinerary is that it qualified for Silver in a single trip!
Good luck
G50