Have to say I generally fly in shorts and t-shirt (LH on BA especially) - keep a long sleeve t-shirt in hand luggage just in case. This is regardless of season. If I find the plane too cold (rare) then I say nothing as I am prepared by wearing less - just grab the blanket (supplied).
Many posters are right - cooling down is limited to amount of layers you start with and your personal take on level of decency...although the earlier mention of sitting around in your skivvies may cause some action on the heat control.

Friend had a group flight to Europe with first two days in London - so outbound was to LHR on BA (a couple of years ago in late November). My main suggestion to him and his son was wear shorts to fly in. Houston was in the 70's so no issue at this end...rest of the 20+ group showed up with their winter coats and sweaters, etc - all s......ing at my buddy & son. Not so much at 38,000 feet. No-one slept but them and their group tours started straight after landing...return flight several others had shorts, purchased from some place in France in early December.

Only time I remember being cold was on DC-10 (showing some age) London to Houston - sitting exit row in WT (pre WT+ days)...feet and legs like ice - neck upwards like being in a sauna. Not a pleasant mix.