Peter Westmacott, Britain’s ambassador to the United States, will host Prime Minister David Cameron this week. (Photo by Marvin Joseph/The Washington Post) (Marvin Joseph/WASHINGTON POST)

At first glance, not much seems changed behind the 10-foot wrought-iron gates of 3100 Massachusetts Ave. since one British ambassador left and his successor took up residence in January. Same Warhol portrait of the queen in the ballroom; same Minton china tea service; same English gardens.

There are new photographs on display — of a dark-haired man eagerly shaking hands with former president Bill Clinton and of the same figure, grayer and more self-assured, standing side by side with President Obama in the Oval Office.

And then there’s the desk — or “secrétaire,” as the new ambassador calls it. (His previous post was in Paris.) It’s a striking addition to the drawing room, decorated in reds and golden-yellow with exotic characters from far-off places. But the most striking thing about it is how well it matches the couches, the curtains and the carpets. It fits in so well, it looks as if it’s been here forever.

A little like Peter Westmacott himself, who has the air — as he eases into his favorite golden-yellow couch (the one he says is best for his troublesome back) — of a man who belongs.

That sense of belonging will be put to the test this week. On Tuesday, just two months after his arrival, Westmacott, 61, will welcome his boss, British Prime Minister David Cameron, for the kind of official visit that an ambassador might expect to oversee once in a four-year tenure.

Cameron will stay at Blair House, eat lunch at the State Department and dine at the White House on Wednesday night. Westmacott says he and his staff “have lots to do to make sure we get that right,” that “nothing is left to chance.” For, as the man who’s reached the tippity top of the British diplomatic tree will tell you, when you are preparing for very big events, little things matter.

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They often matter, says Westmacott, “in inverse proportion to their importance.” So getting ready for a visit like this, he says, which will include talks about the countries’ common military commitments and economic ties, involves avoiding silly slips or, as he puts it, making sure “you don’t score own goals.”

He’s seen breaches of protocol — such as foreign leaders breaking the don’t-touch-the-queen rule — send the British press “bonkers.” In contrast, a “most perfect demure curtsy” that Carla Bruni, wife of French President Nicolas Sarkozy, executed upon meeting the queen in 2008 was the thing that convinced Westmacott that the visit “was going to be a success.”

Overshadowing this prime minister’s visit in many people’s memory is the previous prime minister’s visit in 2009, when it was the home team that scored an own goal.

The gaffe then was over gifts.

Prime Minister Gordon Brown brought Obama framed commissioning papers for HMS Resolute, the ship from which the Oval Office desk is hewn; a pen holder carved from the timbers of HMS Gannet, which served on anti-slavery missions; and a hefty biography of Winston Churchill, who put special emphasis on the special relationship.

Obama reciprocated with a set of 25 American movie classics — which turned out to be incompatible with British DVD players.

And the press had a field day.

A tempest in a Minton china teapot, you might think, but it was just the kind of symbolic gesture onlookers scrutinize for signs of slips in the relationship’s specialness.

Which, Westmacott insists, would be far from the truth: “President Obama made that very clear to me,” he says, “that the United States has no closer ally or better friend.”

Cameron’s can’t be a state visit because the monarch, not the prime minister, is head of state. So the White House, Westmacott explains, has come up with terminology fit for a man who’s not a king: “an official visit with a state dinner.”

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Westmacott chooses his words carefully, pausing to juxtapose easy conviviality with verbal jousting and the occasional polite deflection of a question. (His favorite American food is cheesecake. But which does he most dislike? He begins to answer, then rethinks. Why risk causing unnecessary offense, after all?)

He’s honed these arts of diplomacy over almost 40 years and has been rewarded with a knighthood and plum posts as ambassador in Ankara, Turkey; Paris; and now Washington. He’s so much part of what he does that it’s hard to separate the man from the job. His life is full of pomp, but he’s not the least bit pompous. So how does he let his hair down? He enjoys opera, he says, plays tennis, looks forward to traveling the country to visit the nine consulates he oversees. He wants to ensure that the prime minister’s visit is fun as well as useful — part of that will be the basketball game that Cameron and Obama will attend. But Westmacott can’t give away other details of how the fun will be had. So think of his reticence as the diplomat’s quandary: Westmacott gives information but not gossip; he’s been trusted with big jobs because he knows the difference.

It’s not as if he was born into this role. He’s from “a small house in a small village” in rural Somerset. He heard stories about America when he was a child from his father, a naval officer, who had spent time during World War II in Norfolk, Va., while his ship was under repairs. But the family didn’t travel abroad: “There was no money.”

Westmacott was educated at a private school and then New College, Oxford (“new” because it was founded in 1379, well after the university came into existence), where he was encouraged to try the Foreign Office exam. He agreed to “give it a whirl.”

It’s been a career-consuming whirl with a couple of interruptions, including one in the early ’90s when Westmacott worked for Prince Charles during the period leading to his separation from Diana — a two-year job that turned into three, Westmacott says, because there was “an awful lot going on, to put it mildly.” That “awful lot” invites an awful lot of questions. But this is not the man who’ll spill the royal beans.

From 1993 until ’97, Westmacott served in Washington as counselor for political and public affairs and then spent several years as director for the Americas. He continued to cross the Atlantic, often with his second wife, Susie, an American who grew up in Bethesda and still owns a house here. (Her brother is Hassan Nemazee, a donor to Obama and other high-profile Democrats; he is serving a sentence for fraud, which Westmacott calls “part of the pain the family has to go through.”)

Westmacott spent most of the past decade as an ambassador, first in Turkey, where he led the response to the 2003 suicide bomb attack on the British Consulate that killed the consul general, and then France, where he upturned stereotypes by serving only British cheeses and setting up catwalks for fashion shows at the Paris residence.

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Westmacott says he will support the British food and fashion industries here, among many other British interests, including educational exchanges. At his arrival receptions, he served fish and chips reinvented as cocktail party fare. And on a recent trip to New York to support Britain’s “GREAT” campaign — designed to encourage tourism and investment — he was photographed with British fashionista Victoria Beckham.

Washington feels familiar after the years he spent here in the ’90s, he says, and the visits he’s since made with his wife for family events such as Thanksgiving. The imposing residence, designed by Edwin Lutyens to look like an English country house, is home for now — a home from which he’s preparing not only for the prime minister’s visit, but also the celebration of the queen’s Diamond Jubilee and the London Olympics. Westmacott is opening the doors, rekindling old friendships and making new contacts — and it reminds him of the trait he most admires in Americans: “their breathtaking ability to remember your name.”

“That,” the British ambassador says with a laugh, “is why America rules the world.”