Met James Nesbitt in Emirates First Class Lounge at London Heathrow, he was on the phone at first and I didn't want to disturb him but after he had a photo with my Mum and was up for a chat. Turns out he was on a slightly earlier flight but he seemed really nice and friendly.
Also found myself on the same flight as Jennifer Aniston, she was sat in the seat directly in front of me and was really nice to all the staff and passengers onboard around her despite us all having a bit of jet lag.
Spoke to Mark Austin on board a flight once also, he was flying out to broadcast and he had a chat with my family and I, he was over the aisle but was more than happy to talk. - Seems to be a correlation with celebrities on flights!
Saw into Joey Barton at a restaurant once and you could just tell he wanted everyone to notice him and crowd round him when in actual fact, not many people even realised they were in the presence of a celebrity. I didn't even know who he was until I overheard someone talking.
Sat opposite (other side of the aisle facing the other direction) Rowan Atkinson on a train to London. He was very quiet and reserved, you could tell he didn't want anyone to notice him as he just wanted a peaceful journey which was understandable. If you see him in real life, he just oozes intelligence!