Altough temperature selection in any passenger transport aircraft is set to 24-26 degrees, it does not mean that you feel that it is or that your body is exposed as if you would sit in a room with 25 degrees C air temperature.
The problem in aircraft is that they fly on altitudes where outside temperature is below 50 degrees, so the fuselage temperature is transmitting this temperature into the cabin. In physical terms we talk about heat radiation. That's why you feel colder on a window seat than in an aisle seat.
I'm also sure that in a lot of other places between your home and the aircraft temperatures will be higher than 20 degrees.
I would succest that you take a window seat and ask the cabin crew if they would be so kind to reduce the temperature as much as possible. Good luck
Dani