Obama, who spoke to reporters after wrapping up talks with British Prime Minister Gordon Brown, said he wasn't sure if there would be any immediate political impact from the trip - and that he wouldn't be surprised if there was a dip in some of the polls in the week since he left home. People in America, he said, are worried about gas prices and home foreclosures.
"The reason that I thought this trip was important is that I am convinced that many issues that we face at home are not going to be solved as effectively unless we have strong partners abroad," he said.
"And unless we get a handle on Iraq and Afghanistan, not only are we going to be less safe, but it's also going to be a huge drain on resources."
Obama said he and Brown discussed a wide range of issues, such as climate change, terrorism and financial markets.
"The prime minister's emphasis - like mine - is on how we can strengthen the trans-Atlantic relationship to solve problems that can't be solved by any single country individually," he said.
The Democratic hopeful seemed relaxed as he strolled down to the prime minister's office at 10 Downing St., pausing briefly to shake the hands of two somewhat startled police officers standing near the door.
He turned to television cameras, smiled, waved and said "Hello," before walking into Number 10. Brown greeted him just inside the door.
Pooled television images showed Brown offering Obama a chair on the Downing Street terrace before the pair settled down for two hours of talks.
They later took a stroll in the sunshine around Horse Guards Parade, the vast open space where military reviews, such as Trooping the Color, are often held. Tourists snapped pictures from a distance while security guards walked ahead of the two men.
The walk offered a photo opportunity with a London backdrop for Obama, whose visit to Britain has been decidedly low-key, particularly after the huge crowds he drew earlier in the week in Germany.
Obama later said he had also wanted to take a walk through St. James' Park, but security guards pulled him and Brown back. The park is near Buckingham Palace, home of Queen Elizabeth II.
With the exception of the walkabout, British officials took care to make certain the same photo opportunities were offered to Obama as were given to Republican presidential contender John McCain during his visit earlier this year. Protocol normally dictates that visiting contenders be given the same treatment thereby avoiding the appearance of any favoritism.
Obama stressed the importance of the bond between the two nations.
"We've been through world wars together. We speak a common language. We share a belief in rule of law and due process," he said. "We just like the people. There is a deep and abiding affection for the British people in America and a fascination with all things British that's not going to go away any time soon."
The Illinois senator also offered reassurance to Brown, whose plummeting popularity took another hit this week when he lost a parliamentary by-election in a Scottish seat long held by the governing Labour Party. But Obama pointedly said Brown didn't need his political advice.
"You're always more popular before you're actually in charge," Obama said. "Once you're responsible, then you're going to make some people unhappy."
Obama later met with Conservative leader David Cameron for a postcard moment, posing for photographs in front of Big Ben and the Houses of Parliament.
Earlier, he met with former Prime Minister Tony Blair, who is now a Middle East envoy. The meeting lasted for just over an hour.
Obama is on an election-season trip, financed by his campaign, that ends Saturday with talks with British officials. Part of his goal for the trip through Afghanistan, the Mideast and Europe has been to allow him to make his debut on an international stage in the hopes of reassuring skeptical voters in the United States about his readiness for the presidency.
On The Early Show, Republican strategist Kevin Madden said the risk for Obama on his overseas trip was that some American voters might believe the presumptive Democratic nominee was "more interested in Berlin, Germany instead of Berlin, New Hampshire.
"That's the big risk here. Does he look more interested in appealing to Europeans than the Americans facing everything from rising gas prices and unemployment?"
Mike Crowley, Senior Editor of the "The New Republic," said the images of Obama over the past week will only benefit him. "Barack Obama was a state senator five years ago," he said on The Early Show. "If people think he looks too presidential, he'll take that good news for him.
"He's going to come back, when this trip's over, hammering the kitchen table issues: gas prices, price of food. He's going to let people know he cares about that, [but he] had to get this done, show people 'I can do the world stage and do it well,' and I think he succeeded."
The visit has generated enormous interest in Britain, with the press pack stretching the length of the road outside the prime minister's office. Reporters stood five or six deep in some spots, squatting beneath scaffolding to get a good view.
"We're going to impose some discipline on this mob," Obama quipped before answering questions.
On Friday, he met for more than an hour with French President Nicolas Sarkozy at the Elysee Palace in Paris.