From a pragmatic and logical standpoint, the aircraft should be kept on the cooler side. It is easier to warm one's self up by adding layers than to cool one's self down. Furthermore, the tools to warm up are actually given to each passenger - a blanket. And further to that, the crew are moving about a lot and would be expected to be warmer due to that, and therefore for their comfort, erring on the side of being cooler is rational.

On top of all that, during boarding, people are hauling their hand luggage, lifting it into bins, and many pax might have just sprinted a mile or more carrying heavy bags from another plane... cool planes are great.

It's always extremely irritating when anyone requests the temperature be adjusted to accommodate them. There are hundreds (usually) of passengers on any given long haul plane. It is the height of arrogance to ask that the temp be adjusted for the other few hundred to suit the individual. Mentioning one is hot or cold is appropriate, but demanding or even requesting action seems ridiculously arrogant. The airlines should follow standards for this sort of thing and it should be monitored closely for maximum pax comfort.

I do agree that BA planes are perpetually hot. I only fly long haul on BA and it's one of the few things I dislike about them.

As for T5, I think the place is so damn hot because the people who control the temperature are standing around all day but the people who are dealing with it are moving and carrying luggage and usually wearing outerwear.