5 years ago I packed a Trek T2000 in 1.5 bike boxes taped together and took it with us for free on British Airways from Tampa to London. BA used to take bikes free as sporting goods, but I don't know if they still do. When I asked them on the phone about taking a tandem, they said it would have to go freight, but we took it to the airport anyway, gave it and our bags to a skycap and all the check-in agent said to the skycap about the big tandem box was "Take that to oversized baggage."
When we arrived in London, it was already in baggage claim. Customs were not interested and the bike stays in the UK.