Johnson's problem comes down to one important factor: His dual-citizenship. Despite being very, very British, Johnson was born in New York and lived in the United States until he was 5, hence becoming a natural born citizen. And despite some threats to renounce his citizenship ("After 42 happy years I am getting a divorce from America," he wrote in 2006 after a spat with a U.S. immigration officer), he renewed his U.S. passport just two years ago.
It's certainly tempting to dismiss Johnson's dilemma as the simple case of a very rich man attempting to float (sic - flout) the law ("Come on, Boris!," the New York Times's Roger Cohen wrote this week. "Give us a break.") but there is a degree of sympathy to be found here: American citizenship carries with it a uniquely vexing taxation problem. The United States is one of only two countries where taxation is based on citizenship rather than residence (Eritrea is the other). If Johnson lived in the United States, for instance, he would not have to file a British tax return.
The unusual U.S. policy dates back to the Civil War and the Revenue Act of 1862, which called for the taxing of American citizens abroad, in part to punish men who fled the country to avoid joining the Union army.
In practice, this is usually often just an annoying bit of paperwork for foreigners -- while the average citizen would have to file a tax return, it's unlikely they'll have to pay anything. However, it can become expensive for higher earners, especially when tax laws don't line up. As Lisa Pollack, an American expat herself, explains for the Financial Times, this is what appears to have happened for poor old Boris:
In the U.K., gain on the sale of one’s home is not subject to tax. In the U.S., a gain above $250,000 (for a single filer) is subject to capital gains tax. Also in the US, home ownership is subsidised by a deduction against income of mortgage interest. In short, the countries have different tax breaks on housing.
Johnson's U.S. tax bill for the sale of his home in London is thought to be in six figures. Given that the home is in the country he lives and works in, and he has not lived in the U.S. since he was 5, you can see why he thinks it's "outrageous."