A growing base of Brits is clamoring for a permanent American football franchise in London, but don’t expect the National Football League’s U.K. invasion to happen just yet. Even as buzz around a prospective overseas expansion reached a fever pitch this week, a top league executive made it clear Thursday that there won’t be an NFL team across the pond until officials are positive it’s a logistically viable option.

The mutual interest in the NFL’s increased presence in London is clear, but the league has several obstacles to overcome before it can realistically place a team overseas. NFL officials have to figure out a way to turn the London series from a passing novelty, albeit a successful one, into an attraction that draws fans over a full season of games. The United Kingdom’s tax laws, travel logistics, player union concerns and team revenue are just a few of the league’s concerns. In the meantime, all indications suggest the London game series will continue to be successful, and the league will continue to reap the rewards.

“The NFL, they have an exciting package. The consumers are clearly happy with it. The players association is supporting it,” said Mark Rosentraub, a professor of sport management at the University of Michigan’s School of Kinesiology. “It’s like one of those things: If it ain’t broke, don’t fix it.”

London’s mayor is openly lobbying for a permanent NFL franchise at a new stadium owned by English Premier League soccer club Tottenham Hotspur, with which the NFL already has a partnership. But the NFL's current three-games-a-year recipe seems to be hitting the spot: All three of the NFL’s regular-season games at London’s Wembley Stadium drew sellout crowds of more than 80,000 fans each last season, to the tune of more than $30 million in revenue.

The NFL sees the value of overseas expansion, even if it isn’t exactly sure how to pursue it in the short term. Aside from sellout crowds and the resulting ticket revenue, at least one NFL team that regularly plays “home games” in London has seen a spike in sponsorships. The Jacksonville Jaguars, who began playing in the city in 2013 and will continue to play at Wembley Stadium through at least 2016, saw sponsorships rise by 29 percent after their U.K. debut.

Mark Waller, the NFL’s executive vice president for international operations, personally touted the NFL’s overseas growth as recently as this month, saying the league could play a full slate of eight regular-season home games, split across Wembley Stadium and Tottenham’s new area, within five years. The league is exploring either an expansion of its current system, which calls for various NFL teams to play in London each year, or a permanent relocation of an existing franchise.

That was likely music to Londoners' ears. Boris Johnson, the city's mayor, added to the buzz Wednesday during a Twitter Q-and-A session with his constituents. Asked when the city would have its own NFL franchise, Johnson said he’s “working hard” on a deal for an American football team at Tottenham’s stadium “in the next few years.”

But the NFL has to be careful, said Brian Mills, a professor of sport management at the University of Florida, not to mistake a passing fad for a reliable long-term business strategy. “They sell out these games that come over a couple times of a year, and there’s kind of a novelty effect going on,” he said. “One of the things the NFL needs to be careful about is assuming that, because these novelty-type of games are selling out, that there might be a rabid fan base for a new NFL team.”

That may be what's behind the league's wait-and-see approach. Waller expressed hope in several aspects of the NFL’s situation in London Thursday, including its playing venues, existing sponsorship opportunities and a growing fan base. But the NFL executive also made it clear there was no timetable for franchise relocation – not until the league figures out the logistics.

“The key for us is that we’ve got to build real certainty that any team that was to make that undertaking could truly be competitive,” Waller said during a conference call Thursday, according to the Associated Press. “The last thing we would ever want to do is to put a franchise in a place where the logistics, the travel, the sourcing of players, the infrastructure for supporting teams through a season doesn’t exist.”

An NFL team permanently located in London would face the daunting reality of regular cross-Atlantic flights during the course of an already-grueling schedule of games. So much travel time would cut into a team’s ability to practice and prepare.

The franchise also would have to navigate the United Kingdom’s complicated labor laws and high income tax rates, which at 45 percent for incomes above 150,000 pounds a year, are significantly higher than those in the United States. Critics also have questioned whether it’s realistic to ask a college football player from Alabama to relocate to a new continent to play the same sport he could just as easily play in Miami or Detroit or whether a high-priced free agent will want to move across the pond.

League officials would require the players’ association’s blessing before they could make such a drastic change to the way one of its franchises operates. Given that obstacle, it’s unlikely any relocation will occur until at least 2021, when the league’s current collective-bargaining agreement expires, Rosentraub said. And the NFL will still have to convince a team owner to leave a perfectly lucrative existing market to play overseas.

“It’s hard to imagine what community really wants to divorce itself from the NFL,” Rosentraub said. “There are certainly some teams that are less successful than others in smaller markets, but whether or not that’s in the best interests to have one of those teams [relocate], and if they did relocate, do you solve anything by putting them to London relative to all the commutation expenses.”

With more than $7 billion in revenue split among 32 NFL teams in 2014, there is at least some concern the league has tapped out its growth opportunities in the United States. There are potentially billions of dollars for the taking in European markets like the United Kingdom or Germany if the NFL can work around the logistical concerns. Overseas expansion could lead to global sponsorship and media rights deals.

“I think that’s really what they’re looking at: How large is the fan base, and is it large enough to justify having a team,” said Darin White, a professor and coordinator of the sports marketing program at Samford University in Alabama.

Determining just how many fans of American football exist in these regions, and whether interest in the sport is more than a trend, is part of the problem. While more Londoners are following the NFL than ever before, few actually play the sport, and it’s unclear if they’ll continue to turn out in droves if the slate of games expands from three to eight.