ROYALS FLUSH: HOW DI’S BOYS COULD PUMP UP INHERITANCE: STOCKS, AND TIME, COULD TURN MUM’S $22 MIL. INTO $1.5 BIL.

Cool, Britannia. The $21.8 million the late Princess Diana’s two teenagers inherited last week could balloon into a whopping $1.46 billion in 30 years — if they invest like bulldogs in an aggressive stock portfolio that returns 15% annually. Even a more genteel 8% average annual return would build a $221 million empire for Prince William, 15, and Prince Harry, 13, by the time they reach their mid-40s.

At a conservative 0.5% of assets under management, the privileged financial planner would reap up to $7.5 million in fees annually.

“Sign me up!” shouts Richard Bregman, president of MJB Asset Management in New York, who recently opened his own firm overseeing $7.5 million in assets.

Britain’s Coutts Group, the financial adviser to Queen Elizabeth II, did not return calls. But Yanks weren’t reserved when asked how they’d counsel her grandsons. After all, the boys seem destined for lives of ribbon-cuttings and hospital handholdings, assuming they inherited their mother’s better traits (and Prince Charles, the paparazzi and House of Commons republicans don’t muck it all up).

“This is all gravy — and they’re young, that’s why I’d put it in stocks,” says Vincent Schiavi, a planner in Wilmington, Del.

Invest in the Colonies, he suggests: The Standard & Poor’s 500 stock index is 327% higher than it was in 1986, while the U.K.’s Financial Times 100 stock index is up 239%. But with the U.S. market reaching all-time highs, Mr. Schiavi would tilt toward value plays — and spread 20% overseas.

Dennis Gurtz of American Express Financial Advisors in Washington, D.C., notes load mutual funds often waive sales charges on accounts above $1 million. He’d diversify among 15 offerings, including (surprise!) Amex’s IDS New Dimensions and Blue Chip Advantage.

And if everything tanks? Just wait. Grandmummy is worth an estimated $16 billion. Including the crown jewels.