Hey, We bought ours on a plane when our little one was 3 months old. Flew to edinburgh with ba but didn't have a bag for it. Let us have it right until the last minute. Unfortunately brakes were broken when it came off! Have travelled elsewhere with budget airlines and they have always let us have the pushchair until the last minute, although we never made the mistake of bringing the bugaboo again and just borrowed my sil's maclaren.
Make yourself vaguely known to the check in crew and they should hopefully make sure you sit together, good luck!
I flew with DD at 4 months, we checked in the Bugaboo in it's bag and carried DD through the airport in the baby bjorn. It was so much easier, esp going through security, and left me with 2 free hands. I was given the advice here on MN, and to travel with a sling was among the best advice I have been given.
And somewhere there a few Japanese people who had been on holiday in London and have photos of DD in a sling, as they thought it was really cute, and she loves smiling for the camera.