OK, I'm taking a private trip from London to Ho Chi Minh City return at the back of the plane.

I'm a *G (not via SQ), but actually I don't fly Star Alliance much, I just took a few biz class flights a while back. I haven't flown on SQ for over 10 years (correction: I had a regional J flight with them about 4 years ago).

Is there any advantage of me taking SQ on this trip, given I'll be booking Y, compared to another airline which may be £100-odd cheaper for a similar schedule?

What is the lounge access policy in LHR, SIN and SGN and which particular lounges can you use? I've Googled up a few threads which suggest the lounges for non-SQ *G are not really that good or worth making any effort to visit?

Finally, can you do paid upgrades to J at check-in on SQ? (Or I always welcome any OpUp which comes my way, but my reading on SQ suggests OpUps don't happen). My return flight is overnight, so I'd consider upgrading it depending on the cost and how much sleep I've had in the days beforehand