From #FlyerTalk's 'Boeing77':

​​​​​​https://www.flyertalk.com/forum/british-airways-executive-club/2054986-what-s-typical-temp-setting-b787-3.html"

"On the flight deck we set a master temperature been 18 and 29 degrees (why you’d ever want it that warm?!). The cabin crew can then adjust that temperature by +/- 3 degrees.

As others have said the service standard is 21 degrees. On the whole I’m not aware of crew increasing cabin temperatures after the service etc, it is something that I keep an eye on when I take a look at the air conditioning schematic throughout the flight. What I have noticed however is the commanded temperatures in WTP and WT are usually higher than CW and F.

Body temperatures fluctuate with time of day/circadian rhythm etc. I’ve been asleep in the crew rest area, set to 20 and fallen asleep easily, only to wake up far too warm.

In my five years on the 787 I’ve never seen a technical issue that would impact temperature control. The system tells you the commanded temperature compared to the sensed, and I would say it holds it incredibly well."

Others have covered the cabin service standard regarding temperature on the forum (and plane types).