And yet, many of the Kensington congregation (and of the Russian-singing choir) are not. Even some of the priests are British. Bernadette Sharpe, a nurse, found the music "much more spiritual" than anything she had heard before. Freke de Graaf, an acupuncturist, was attracted by the "joy" of the church. "It's really alive. It's not just a Sunday church. And the tradition of the theology is rooted very deep," he says.
While the Catholic and Anglican churches constantly seek to reinvent themselves, reforming the content and language of services, the Orthodox church - in effect, a group of churches - finds its strength in standing still.
It would seem to be the very antithesis of modernity, priding itself on its refusal to follow the fashions of the rest of the world. In some respects, that is precisely what has ensured the Orthodox church strength today - not just in the countries where it is rooted, but also in Britain at the end of the 20th century. No modern fads: just the church, the music, and God.
The art is part of that deliberate lack of modernity. At the Royal Academy in London, an exhibition opens today which pays homage to the the simplicity and complexity of Russia's religious art. In the West, the link between religion and art has been almost severed. We expect to see a Madonna and Child in an art gallery, not in a church. At an Orthodox service, by contrast, small icons are dotted around the church, and the large central iconostasis forms a focal point.
The art has a severe beauty which stands in stark contrast to the lushness of Western art. None the less, the austerity is never removed from earthy reality. In Andrei Tarkovsky's classic film Andrei Rublev, about the greatest icon-painter of all, the two themes are powerfully merged: the painful realities of the artist's life give way in the final minutes to the hitherto unseen glories of Rublev's art itself.
For many, the Orthodox church is the end of a long spiritual search. Father Michael Fortounatto, one of the Russian priests in London, describes a not untypical phone call last week from a woman who was keen to be admitted to the Orthodox Church. "She grew up an Anglican. She's been through India, and meditation. Now, she has decided to come to us." As the 20th century gives way to the 21st, the number of converts is growing.
Deacon Joseph Skinner, himself a former Catholic, talks of the importance of ceremony. "In the Orthodox church, there is a sense that people are standing before the presence of God. Even in a church as ancient as the Catholic Church, something very precious has been lost. It has become introverted, on man and his world." He insists, however, that this is "not world-denying - it's profoundly world-affirming. We have the best of both worlds - our feet on the ground and our head in the heavens."
Peter France, former presenter of the BBC's religious current affairs programme, Everyman, is another convert. He has recently published an account of his voyage from agnosticism through hesitant belief to the certainties of the Orthodox church. He talks admiringly of the "combination of matter-of-factness about the ceremonies and a high seriousness of purpose".
The composer John Tavener - who gained a new dose of international fame when his Song for Athene was performed at the funeral of Diana, Princess of Wales, last year - is one of the best-known Orthodox followers in this country. He sees the Western tradition as deeply flawed: "I'm not very at home with humanism." He emphasises the idea of "not-knowing" instead of rationalism, and argues: "We don't know. We cannot judge. Only God can really judge what's going on inside a person's soul."
Until recently, the Russian congregation at the cathedral in Ennismore Gardens consisted only of a few emigres. Now the number of Russians at Orthodox services in this country has increased with diplomats from the Russian embassy - official representatives of the Soviet Communist government just a few years ago - coming to have their children baptised, and to receive baptism. Even now, however, Britons make up as much as a third of the congregation. English-language leaflets at the cathedral encourage visitors.
Elsewhere, the importance of the church as an exclusive bearer of national identity (foreigners not welcome) is strong. The Serb Orthodox Church has often been remarkably close to the nationalism of the Serb leader, Slobodan Milosevic. In Russia, the identity of church and state has been equally strong: during the Second World War, Stalin lifted a ban on the Church to mobilise it on behalf of the Soviet motherland.
During the Soviet era, the Church worked hard to achieve a kind of accommodation with the state. Church leaders avoided public criticism of the authorities. In return, they (though not their congregations) were allowed a relatively comfortable life, though constantly under the beady eye of the KGB. The strange cohabitation led to a split in the church, with a separate emigre Russian Church in London - a split which persists seven years after the collapse of Communism.
In Russia today, the Church is more important than ever. The powerful mayor of Moscow, Yuri Luzhkov, has made the rebuilding of the Church of the Saviour in central Moscow (dynamited by Stalin in 1931) one of the most high-profile and ambitious items on his agenda for the New Moscow. The rebuilding of the church is seen by many Russians as proof of Russia's intrinsic greatness.
The art which goes on display at the Royal Academy this week represents the spiritual, non-national side of the Orthodox church - the side that has attracted so many supporters in this country, as they seek to break away from the perceived wishy-washiness of much Western religion. Tavener emphasises the spiritual qualities of Orthodox worship and art alike: "When I see a Renaissance painting of the Mother and Child what I see is a fat Italian baby, wonderfully executed. Such a picture conveys no theology - which an icon will always manage to do because it is not striving to be realistic."
With this "back-to-basics" quality, it sees itself as the core church from which the Catholics broke away, just as the Protestants later broke away from Rome. In other words, no zigzags here. As Peter France notes in his Journey: "The Orthodox church could never be accused of being trendy." In some respects, that is true. None the less, even as fashions come and go, the confident tranquillity that the Church offers is more in demand than for many years. The Church scarcely changes, just as the art has changed only imperceptibly over the years. It does not need to go to the people; the people come to it.
The Art of Holy Russia at the Royal Academy from 19 March to 14 June, 10am to 6pm daily, with extended opening to 8.30pm on Friday.
Peter France: Journey: A Spiritual Odyssey (Chatto & Windus, pounds 12.99)