LONDON — The wedding of Prince Harry and Meghan Markle in May 2018, watched by a television audience of almost two billion viewers worldwide, was heralded by many as the ultimate 21st century fairy tale. It was hoped that the union of a beautiful biracial American actress and one of the world’s favorite princes would firmly nudge the British royal family into a new era.

It was also hoped, somewhat less romantically, that the potential star power of the Duke and Duchess of Sussex (as the new celebrity couple became known), would be an economic windfall for Britain, potentially bringing billions of pounds to the economy for decades to come.

Just eighteen months later, the couple announced via Instagram that they would be stepping down as “senior” royals, splitting their time between Britain and North America and working toward becoming “financially independent.”

Just how much of a loss for Britain, its monarchy and businesses, could the new arrangement represent?

“The short answer is that it is rather hard to say, given no one yet knows the exact terms of Megxit,” said David Haigh, chief executive of brand valuation consultancy Brand Finance. The company calculated that the wedding of the Duke and Duchess of Sussex generated £1 billion for the British economy in 2018 alone, including an additional £300 million in travel and accommodation spent by foreign tourists and £50 million spent on merchandise such as coins, dish cloths and clothing, not to mention those billions of clicks in free media coverage that enhanced the value of the Britain brand.

In the months since the wedding, patronage by the Sussexes has continued to generate hefty revenue for retail and leisure industries in particular, for British household names such as Marks & Spencer and John Lewis and through charitable projects like the Duchess’s fashion workwear collaboration with Smartworks.

A new half-in, half-out of the firm arrangement would not necessarily put a stop to those earnings and endorsements for Britain and its businesses; if anything, extensive ambassadorial work at home and abroad could even increase them. But it all depends on the nature of the long-term relationship with Buckingham Palace, and what, with the creation of the Sussex Royal brand, the couple plan to control themselves when it comes to their cultural and commercial influence.

“Harry and Meghan say they want to be involved in Commonwealth activities, patronages and support the Queen — which suggests retaining some form of official royal role and duties,” Mr. Haigh said of the Sussex’s unveiling of what they termed on their website “a new working model.”

“But they also want to earn their own money and partly live in North America. So, in other words, do whatever the hell they like, no matter what has been invested in them by others,” he said.

Those outside investments include security at their wedding, which was paid for by British taxpayers and cost an estimated £30 million (the church service, flowers and reception were paid for by the royal family) and £2.4 million to renovate the Duke and Duchess’s official residence, Frogmore Cottage in Windsor, which the couple moved into nine months ago. Until now, five percent of the couple’s official expenses were paid for by the Sovereign Grant — an annual lump sum from the British government — and public funding the couple say they now plan to relinquish.

However, they also said via the Sussex Royal website that they hope to continue receiving money from Harry’s father, Prince Charles, who funds the remaining 95 percent of the couple’s personal and professional expenses through his private estate, the Duchy of Cornwall, which he possesses by right of being the Queen’s heir. In 2018, Prince Harry and his brother William, the Duke of Cambridge, and their wives received £4.9m from Prince Charles and the Duchy. This contributed to Prince Harry’s fortune, believed to be around £30m. Meghan’s net worth before marrying Prince Harry was estimated at £3.5m.

Whether Prince Charles — an advocate of streamlining the size of the monarchy, which in line with the downsizing trend taking place in other European royal families — will reconsider that support remains to be seen. At a time when there has been greater scrutiny around the degree of public spending on the royals who qualify for police protection, Harry and Meghan also noted on their site that they will continue to have a publicly-funded security detail as they embark on a quest for financial independence.

The Duke and Duchess “value the ability to earn a professional income, which in the current structure they are prohibited from doing,” said a statement published on their website. (Royal protocol states that no gifts or funds, including hospitality or services, should be accepted that would, or might appear to, place a royal family member under any obligation to the donor.)

But what might these new livelihoods and income streams entail? And will the family have any say?

Last June, the Sussexes submitted a trademark application for the Sussex Royal foundation on more than 100 items, including hoodies, socks, textbooks and bookmarks. Rather than just selling Sussex swag, the more likely road to (greater) riches for the former soldier and actress could hinge on book deals, speaking engagements and select brand endorsements.

“Given Meghan’s background, social network, and the world she comes from, it wouldn’t surprise me if they moved in a more familiar direction of celebrity should they get approval from the Queen,” said Kenya Hunt, the fashion director of the British magazine Grazia. “Fashion would also be a natural arena for Meghan, given her industry relationships on both sides of the Atlantic. This news has been very dramatic — people are shocked — but whatever happens with the palace, there will still be a group of fans who admire her style.”

Whatever the Sussexes choose to do, Mr. Haigh of Brand Finance said he did not expect it to hurt the long term clout of the rest of the royal family.

“The royal family generates at least £2 billion of annual incremental revenue to the British market, from promoting endorsements and royal warrants to bolstering soft power; Kate and William were driving that long before Meghan arrived on the scene,” he said. “The growth thanks to the next generation may be less than was hoped for after this, but I doubt the cash flow will actually decline. ‘The firm’ remains too strong an institution.”