I just went and took a look at the forward facing car seat that my parents had for my daughter after she grew out of her infant car seat (for some reason its being stored in my dining room ) Graco Coast, says it's international, (number given on base is for Canada) but from the instructions to install on the side needs the shoulder belt so not for airplane use, next time i go in somewhere that sells car seats i'm tempted to ask!

I don't think i've ever flown in the UK itself, its often quicker and cheaper to take the train unless your journey (by car or train) would take over 4 hours

I don't know much about Australian car seats, i do know if you take a car seat with you to Australia intending to use it it must meet their regulations, gotta be annoying to take a car seat all that way on a plane to find you can't use it at the other end