If you're not getting out, I'd strongly recommend stopping in SIN if you can.

The SIN lounge is easier (though not "easy") to find than than in BKK; if you're a Gold card holder the SIN lounge is much bigger, but the J lounges are roughly comparable, though I still prefer the SIN J lounge (joint with Qantas).

In BKK you have to go through security again before going upstairs to the lounges and boarding gates, while in SIN you go through security at the gate just before you get on board (in case you're buying lots of Duty Free liquids, this is important to remember).

SIN is better air conditioned, so much less humid which makes for a more comfortable transit.

The shops are better in SIN (though you'll have hardly any time to do any).

Main thing is to march as quickly as possible to the lounge and get a shower in during your brief transit; take mini talc to help dry off (the cubicles are humid!), and mini conditioner if you need it!

Don't be put off by a short transit time; there's always enough for a quick shower, but do make sure your watch is correctly set to local time to avoid confusion about re-boarding times.

I'd also recommend taking a small feather pillow/cushion in an Egyptian cotton pillowcase; the BA J ones are polyester filling and far from comfy, even if you can snag two. It's worth it on such a long flight, and really helps you get proper shuteye.

Book a seat on the 747 Upper Deck if at all possible (the only reason I'd ever choose to go via BKK was for the 747 plying the route; now BA isn't going onwards from BKK you'd probably have to switch to Qantas (QF) anyway)!