Sorry for the delay in replying....my t'internet connection has been horrible for the last while, and I'm struggling to stay online
Stay away from the sedatives...they won't accept sedated animals on flights. A sedated pet finds it hard to control their body temperature, and whilst its not freezing cold/roasting hot in the hold, the temperature can fluctuate a little and the animals in the hold need to be able to react to this and regulate their temperature.
With regards to toiletting in the crate..sadly, if they need to go, they need to go. But you may well find they hold it just fine.
The longest flight I ever sent a dog on was 20+ hours to Singapore....I fretted and fretted, and made myself sick with worry (it is inevitable, no matter how muchItry to placate you I know you will worry)...too long in the crate etc etc....but the handler I had engaged to meet the dog at the other end said that he bounced out of the travel crate, once cleared through customs, happy as larry...and cleanas awhistle. His first port of call was to christen the nearest Singaporean lamp-post though
All you can do, especially on the longhaul flights...its use proper vet-bed (water drains straight through it keeping the fluffy toplayer dry & comfy) and line it with ample newspaper underneath.
Selotape (or marker pen) your cat's names on their individual boxes, so that the handlers can talk to them using their names when they are being shuttled on and off the plane(s) - the handlers I have spoken to tell me this is a big help to them because they are all animal lovers too, and like to know their names, and it helps to keep the pets happy too.
All IATA flights, approved for carrying pets etc are well versed in handling animals, and all the staff & handlers are well-trained and sensitive to the animals needs & fears.
I know you will worry, no matter what I say....all I ask is that you try not to worry too much, and I'd place good money on betting your cats will travel with absolutely no problems, most likely sleep soundly through almost all of the flight....and when they get out the other end, after clearing through customs, they will not have the faintest idea why you (their human) is most probably crying with happiness & relief, whilst hugging them hard!
With regards to Heathrow/Gatwick...I only had a quick look at approved pet-carrying flights between Japan & UK on the Defra site, but they all seemed to go to either Heathrow or Gatwick...so you will have to choose which one of those you'd prefer them to come into.
Here's a handy page from the IATA site, with regards to flying with pets:
https://www.iata.org/whatwedo/cargo/live_animals/Pages/pets.aspx
And if you want to get into the nitty gritty of what the airlines have to do to become approved pet carriers, this page might help allay some of your fears
https://www.iata.org/whatwedo/cargo/live_animals/Pages/index.aspx
