What to wear on the plane

I am going on a holiday of a lifetime to Australia with a stopover in Hon Kong on the way, and I don't know what to wear on the plane, I am worried that whatever I wear in England will be too hot when I get to Hong Kong plus the nurse at the health centre told me the Mozzies bite during the day and they really like the taste of me, so would need long sleeves! Is the temperature in Hong Kong really humid? I am not a seasoned traveller and have been saving for this for 5 years any advice is welcome!