My experience is that BA is in the better part when it comes to European ones. I always have a small travel-thermometer. Some times it tends to go above 24C (esp. upper deck 747), but when I ask them to lower it they are always happy to do that. A few times they have even asked me up front - "do you think it is to warm here". My low-record on on BA (747 F-cabin) was 19C (without asking). First time I could really enjoy the duvet.

Worst experienced was on LH A380 F when it was new. They did not managed to get it below 27C! SAS also have some issues with their A340/A330s where they are unable to get it below 25C in the C-cabin. Shorts and t-shirt is standard dress-code for me on those flights.