I've taken a fully assembled, unboxed touring bike from Denver to Europe and back (Scotland, Tuscany, Sicily, Ireland, Tuscany) 5 times since 2004. 4 times involved a British Airways flight from Denver to London. No problems on any of those journeys. Turn your handlebars sideways, take off the pedals and lights, lower the saddle and show up at the check in counter early, and you will probably be fine. Strictly speaking, BA requires you to have a "protective box or bag", so it might not hurt to have one of these: http://www.wiggle.co.uk/ctc-plastic-bike-bag/ handy in case they get sticky about the rules. On my last journey I got the bike on the plane for DIA-London without the bag. For the next 3 flights (London-Bologna, Pisa-London, London-Denver) when they said anything about a bag I just said "but I flew here from Denver without a bag, why do I need one now?". They might get sticky about making you deflate the tires, sometimes I can get around that, sometimes not. There's no real reason to deflate the tires, but try telling that to an airline employee. I figure it's not a good idea to argue too much with an airline employee right when I'm handing them my bicycle, so I usually don't press my luck too hard on deflating the tires.