As most longhaul BA aircraft are not specced with individual air vents, I carry a small (£2 coin size but square) thermometer from eBay, in my bag of wires / power adapters. If it exceeds 21.9 I politely ask for the temperature setting to be turned down to whatever is needed (which may bear no resemblance to the actual achieved temperature) to achieve 21 in the passenger cabin, commenting that I understand that the air flow patterns are not uniform within the aircraft, and vary from aircraft to aircraft of the same type, whilst showing the thermometer display. ETA: Personally I’d love it to be 18 but that’s subjective.

I wait for 20 mins post take off to avoid the “it takes time after takeoff” reply. Any further fob off at that point elicits a complaint on BA.com - not that that has ever achieved anything other than a generic apology email.

Also useful evidence in certain hotels with those ridiculous PIR activated air con controllers that turn off the air con if motion is not detected - eg at night. Sometimes that can be overridden in the setup menu of the thermostat.