Last year, when the International Monetary Fund published a paper on offshore tax havens, it included Switzerland, the Cayman Islands, Jersey, the Bahamas and several other countries where the rich stash their money beyond the reach of grabby governments back home. But there was one surprising new entry on the list: the United Kingdom. The IMF was merely recognizing what wealthy foreigners in Britain have known for years. While British citizens shoulder taxes of up to 40%, residents who weren't born there can take advantage of the nondomicile — or nondom — rule, which means they're only taxed on income made...