United MileagePlus members can earn up to 35,000 bonus miles on nonstop flights from the U.S. or Canada to London on United or Air Canada through Dec. 12, 2014. To be eligible for the promotion, you must first register at http://insideflyerus.wpengine.com/link/?11748 . The offer is only valid on tickets purchased after Sept. 14, 2014.

Members will earn bonus miles for each roundtrip flight up to a maximum of 35,000 bonus miles for three roundtrip flights: earn 7,500 bonus miles for one roundtrip, 10,000 bonus miles for two roundtrips and 17,500 bonus miles for three roundtrips, for a total 35,000 bonus miles. You must fly nonstop between the U.S. or Canada and London on fare classes W and higher on United or Air Canada to get the bonus miles.

Bottom line: Air Canada is offering the same promotion for Aeroplan members, so if you prefer to earn Aeroplan miles instead of MileagePlus miles, register at http://insideflyerus.wpengine.com/link/?11749