Flying used to be an intimate affair.. you dress up in your best clothes and look fresh for the part.. then came the american cheapie flights.. and then the backpackers.... that's ok. As long as you 're in the back...
For First Class travel, I wouldn't go in shorts. I mean you can wear anything you want, but i consider flying still a private matter. I would frown upon anyone wearing a tank top and shorts, slippers and all for I view that you are flying a space with other people about. If you're in your own private jet, sure go naked for all anyone cares. But in a cabin, best to wear something decent.
Besides the cabin can be a tad chilly. So dead in the middle of summer or not, cabin temperatures are regulated to be cooler. You may wear your jeans onto the aircraft and then change to shorts for sleeping inflight.. but some see shorts akin to undies. There are ladies about, thank you!
Some airlines once upon a time ago, like BA, would lightly suggest you put something on so as not to offend other passengers. I don't think CX would do that, but best you put on your best behaviour at least.
Its good manners, and it shows good upbringing and you will be treated as such. That's my take.
Try asking this on the BA forum and see what they say !!