Chicco Trio, fab! Carrycot (and carseat) clicks onto buggy, buggy is lightweight and umbrella fold, we took it to US when DS 3 months...Im the only one of my baby friends who hasn't subsequently had to go out and buy a maclaren type stroller as original too big. My friend has a Jane and complains that it is v. bulky
Buggy seat lies flat also so for your actual flight you could check in the carrycot and use thte buggy at the airport.
I used mine for DS to sleep in overnight when visiting rellys. You can use carrycot int eh car too as there are straps etc for fixing to belts but it isnt as safe as a proper car seat.
As an added bonus not everyone and his dog has one.
An extra thought, have you seen the samsonite pop up travel cots...i had one and was great for when DS small as folds up v. small, i also used this lots when away, was fine until he was about 7 months old, if you used one of these maybe you could get away without a pram bit at all.
Have you travelled with your others when babies?? I would say 8 week old likely to sleep most of flight as for older ones im afraid i have no idea as my DS only 1 but it will only be a shortish flight and maybe the novelty factor will help??
If you live near York you could borrow my pop up cot as im not using it at the mo.