My cat is 13 and flighty and extremely difficult to handle for everyone. I have been told they won't sedate him and I may try something myself but he is very difficult.
The airline won't accept any cat who's had any sedation whatsoever. It's a health risk to him.
You could try herbal things (I've used Feliway in my house before) that act as a general "life is cool" agent (so spray it on the mat he's on in the crate), but for his safety, don't give him anything internally as it's likely not designed to work 1) at altitude 2) for long enough to be worth it (you don't want it working for the first couple hours of the flight and then your cat panicking when it wears off).
My cat is pretty laid-back, but hates time in his carrier, so a trans-atlantic flight was not his favourite thing. He handled it really well, though. They keep the cargo area where the animals are kept climate-controlled and pressurized, just like the cabin. It's much nicer down there, though, as thanks to all the baggage in the cargo, it's quiet, and they keep it dim for the animals so they're encouraged to just go to sleep.
If you can get him into the crate before he leaves, the airline shouldn't need to take him out unless there's an emergency. Animal Reception on the other end will, obviously, to check his details, but by then he'll be in the UK.
