A new chief rabbi, a national centre boasting appearances by some of the country's most admired cultural figures, and a BBC2 history of the Jews by Simon Schama – the Jewish community is celebrating New Year in good heart. Ned Temko surveys the huge changes of recent years

Head north-west from central London, past Lord's Cricket Ground and St John's Wood, and a mile or so farther along, you'll find a new, low-slung, glass-fronted building tucked just off the Finchley Road. If your eyesight isn't perfect, or if you're paying more attention to your driving, you could be forgiven for assuming that it's just an office building, or even for missing it altogether.

Yet the modest sign on its third-floor window announces what could prove to be a dramatic new departure for the Jews of Britain: JW3 – the London Jewish Community Centre. Nearly 10 years in the making, at a cost of £50m, and named for the postal district (NW3) in which it sits, it is due to open this month.

Its initial menu of nearly 1,000 events features well-known figures including Kevin Spacey, Nicholas Hytner, Zoë Wanamaker and Ruby Wax, as well as the former editor of the Times, James Harding, who is now head of BBC news.

These are heady days for British Jews. On the religious calendar, they are emerging from the Yamim Noraim – the "Days of Awe" beginning with Rosh Hashanah and culminating with the fast of Yom Kippur. They're starting the Jewish New Year with a new chief rabbi, Ephraim Mirvis, after his predecessor, Jonathan Sacks, was given a communal send-off replete with tributes from assorted prime ministers, fellow faith leaders and Prince Charles.

BBC2 has just begun airing a new series called The Story of the Jews, narrated by the acclaimed – and unabashedly Jewish – historian and broadcaster Simon Schama.

Yet the significance of – and, according to its founders, the need for – JW3 is that the Jewish community of Britain has changed seismically in recent years.

For one thing, it has grown much smaller. The numbers have shrunk by nearly half since its high-water mark immediately after the second world war, with tens of thousands of Jews marrying out, or just opting out, of the faith, while others emigrated to the new state of Israel.

Despite their major impact in areas such as the professions, science, culture and the arts, the Jews of Britain now comprise a grand total of some 260,000 souls – less than 0.5% of the population. Outwardly, they are more self-confident, especially younger Jews who have grown up in an increasingly multicultural Britain.

But you need only to have visited any synagogue for Saturday's Yom Kippur observance – with young security volunteers checking each arrival and patrolling the exterior – to be aware of an abiding, post-9/11 concern over the possibility of anti-Jewish vandalism, or worse.

Amid the controversy surrounding Israel's stalled peace process with the Palestinians, some Jews, especially university students, have also found campaigns such as the push for an academic boycott increasingly unsettling. Whatever their own views on Israeli policies, for many Jews on British campuses, "anti-Israel" invective has sometimes come to feel not a lot different from antisemitism.

Still, the main shift for British Jews is that there is no longer just one Jewish community, but a mosaic of several, in some ways divergent, communities. The only group increasing in number is the charedim, the strictly orthodox. They are still a minority within a minority, accounting for about one in seven British Jews. But their traditions and practice, and their high birth rate, have insulated them from the demographic buffeting experienced by the rest of British Jewry.

They have preserved not just the customs and clothing of the old eastern European shtetl, but a tight, inward-looking sense of themselves. Ever wary of secular Britain, the rabbis of Stamford Hill in north London, the focal point of charedi life in the capital, recently set up a hotline to invite calls on "breaches of modesty" in behaviour or dress. They're almost equally reticent about contact with the mainstream of British Jewry.

But changes in Jewish life in Britain go deeper than the divide between the charedim and the rest, says Stephen Miller, emeritus professor of social research at City University and a leading analyst of trends in Jewish identity. In the 1990s two-thirds of Jews affiliated to a synagogue were members of mainstream orthodox communities grouped largely under the umbrella of the United Synagogue, the body that picks the chief rabbi. Now, the proportion is barely 50%. Some of the decline is because synagogal movements to the religious left of the orthodox – Masorti, Reform and Liberal Judaism – have been growing, if modestly.

"Yet the basic structure of Jewish identity has transformed itself," says Miller. "In the 1980s and 1990s, British Jews differentiated themselves largely in terms of their level of observance. This was the single, best predictor of how strongly Jewish they felt. Now, that link is far weaker. Many of those who regard themselves as strongly identified Jews have little or no connection with religious practice.

"They may identify ethnically, culturally, socially or through an engagement with Israel; they may describe themselves as 'secular Jews'. But the research shows their sense of belonging and pride in their Jewishness are, on average, not very different from their more observant counterparts."

The good news for those who have feared for the very survival of the non-charedi community – rabbi Sacks himself, who took office in the 1990s, wondered publicly whether "we will have Jewish grand children" – is that there are signs of new life there as well. Old-style British Jewishness used to be done quietly. Synagogues were, for many, as much about tradition or habit, as active religious involvement. Now, it is a rare synagogue that does not have a programme of Jewish learning, whether for children or adults. And where British Jews once aspired above all to blend in, many are sending their children to a growing network of Jewish day schools.

That, says Benjamin Perl, an Israeli-born businessman who settled in Britain in the 1970s, may be key to sustaining a fabric of Jewish community life. In addition to helping establish a Jewish secondary school, he has been central in setting up a dozen new primaries in the London suburbs. Though orthodox himself, he also wants to bring the rest of the community into Jewish education. He estimates 65% of Jewish children now go to Jewish schools. But the figure is boosted by the far higher rate among observant orthodox families. "My aim is not just to help set up these schools as Jewish schools," he says, "but to make them the best state schools in the areas where they're located."

But what of the Jews who don't go to synagogue? The "cultural" Jews. Secular Jews. Or, in a famous quip from the cultural polymath Jonathan Miller, those who insist they aren't Jews at all. Just Jew-ish.

That, at least in part, is where JW3 hopes to make its mark. It is not ignoring the already committed. Its inaugural programme has a rich mixture of Torah and Talmud sessions, debates on Israel and other communal staples. But there will also be comedy nights, jazz sessions, dance and fitness classes, even a taxidermy workshop – after which there will be time for socialising in a kosher restaurant run by proteges of the celebrated Israeli-born chef Yotam Ottolenghi.

But with its aim to reach out further, what is most striking – and in British community terms, most audacious – about JW3 is the explicitly American Jewish model on which it is based. The project was the brainchild of the philanthropist Dame Vivien Duffield, and she took her inspiration from a crown jewel of Jewish community life in the most vibrant diaspora on earth: the Jewish Community Centre on the Upper West Side of Manhattan. She and her fellow funders scaled back on some of the New York bells and whistles – they had planned a swimming pool and health club. But the West Side ethos, the comfortable exuberance of New York Jews, has survived in JW3. The very name, taken from the Finchley Road postcode, is a mix of playfulness and a bid for street cred.

"Our Jewish community is so British," says Raymond Simonson, the 40-year-old CEO of the new centre. "We have always looked at our American cousins as being a bit gauche, a bit loud." But JW3 will, he hopes, "turn up the volume".

JW3 is about opening up, and opening out, he says. "We have tended to keep behind closed doors. We build buildings with high walls."

He hopes to bring in not only the widest range of affiliated Jews, but others. "People who aren't going to synagogue. People who may have married non-Jewish partners. People who haven't been involved in anything Jewish since they were teenagers." People who have stayed away because, in his words, they may have feared "they would be judged".

The sign on the window – the one so easy to miss as you weave through the traffic on Finchley Road – is only temporary, he adds. Awaiting formal council approval is a bigger, bolder, permanent one that will decorate the complex's glass perimeter wall: "JW3 – the New Postcode for Jewish Life."

Ned Temko was editor of the Jewish Chronicle from 1990 until 2005