Arthur J. Gallagher & Co.'s purchase last week of London-based brokerage Giles Group of Cos. underscores large U.S. brokerages' need to grow overseas.

The $362.1 million deal, expected to close in October pending regulatory approval, expands Itasca, Ill.-based Gallagher's footprint in the United Kingdom. Gallagher is the third-largest broker of U.S. business, according to the 2013 Business Insurance ranking of the top 100 U.S. brokers.

“It more than doubles their office locations in the U.K,” said Timothy J. Cunningham, managing director of Chicago-based investment banking and consulting firm Optis Partners L.L.C.

He said Giles' business mix blends nicely with Gallagher's diversified corporate structure.

“Giles has mid-market, retail, program, wholesale, underwriting and personal lines,” Mr. Cunningham said. “It looks like a smaller version of Gallagher.”

In a statement, Gallagher Chairman and CEO J. Patrick Gallagher Jr. said there are similarities, but limited overlap in their respective geographical businesses in the United Kingdom.

“We continue to look for outstanding international partners that have a similar growth strategy and operating structure, and this is what we found in Giles,” Mr. Gallagher said in the statement. “Together, we can significantly expand our operations in England and Scotland as well as add operating platforms in Northern Ireland, Wales, Isle of Man and the Channel Islands.”

The Giles acquisition is not Gallagher's first large overseas acquisition. In 2011, the company spent $158 million to acquire London-based HLG Holdings Ltd., the holding company for retail brokerage Heath Lambert.

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Mark Dwelle, an insurance analyst at RBC Capital Markets, a unit of RBC Securities Inc. in Richmond, Va., said international expansion makes sense for Gallagher.

“They are already well-represented in major U.S. markets. And if they want to continue to grow their business, the most attractive spots are international markets.” Mr. Dwelle said.

Indeed, in a move to expand its market share in the northeastern U.S., Gallagher announced the $276.5 million purchase of Short Hills, N.J.-based Bollinger Inc. in August.

John Wicher, principal at John Wicher & Associates Inc. in San Francisco, agreed that a U.S.-centric business mix would impede Gallagher's prospects for long-term growth, adding that its international revenue has doubled since 2010.

“The U.K. offers them a platform for growth, and London gives them a window on the international insurance market,” Mr. Dwelle said. “So, it's a good platform for international expansion into areas such as Latin America” because London is a hub for international business.

As Gallagher becomes more comfortable managing international operations in the longer term, it may follow competitors such as Aon P.L.C. into more emerging markets, Mr. Wicher said.

“It's hard not to be in Brazil. It's hard not to be in South Africa,” he said.

Likewise, Mr. Dwelle sees the Giles deal as a test that may pave the way for more international expansion.

“For Gallagher this is probably the most appropriate first entree to develop a platform with international exposures before trying to leverage this in less mature markets,” Mr. Dwelle said.