Rich people can be resident in Britain without being domiciled, meaning that they only pay tax on income earned in Britain. That's not the case in other developed countries. Christian Kälin of Henley and Partners, a firm that helps wealthy people to move, describes this as “probably the biggest tax loophole in Europe.” The government occasionally mutters about closing it. But it is aware that London's 1,500 wealthy non-domiciled residents—who, according to BCG, are collectively worth £28 billion—would catch the next private jet out. So it has resisted. The presence of the “non-doms”, plus the success of foreign-born entrepreneurs, explains why five of the ten biggest fortunes (on Mr Beresford's numbers) belong to immigrants.