Any advice on taking our two pet cats from UK to Malta?

You have to take them as cargo - cannot put them in the cabin with you like on inter Europe flights.  This is UK crazyness.

So you have to get a right and proper IAATA approved holder for each cat (specific size) and then fly them with a airline that will do it - Thompsons is one, costs about 250/cat.

Here you have to then meet with the gov vet who gives the cat the once over.  It's fairly easy, I got a agent to do it and it cost me 900 for everything which is not the cheapest option.  Obtaining a Pet passport including all the vaccines and whatnot will be another 200 to 300 depending on how up to date your cats are with their injections and such

But I believe this is overall less tressful than a car.  Cats in a car is a nightmare - where do you sleep with them, smells, etc, reading up about cats and cars its not a good option.   Depends on the cat of course I once had one I took everywhere with me even to work, she was used to it so moving 1500km away was easy, but that's definitely the outlier.