Of all the national stereotypes I have read on MN this is the silliest and least accurate. The UK may be many things, but insular it is not. On the contrary, it's an exceptionally cosmopolitan place.
First of all, the British share a common language with the US, Australia, Canada and New Zealand. So Brits read American novels, watch Australian TV shows, and so on. A Romanian, for example, who only speaks Romanian might watch stuff with subtitles or read translations, but he isn't plugged into a vast transnational culture in his own language.
Second, the UK has always had a large immigrant population. And I don't just mean people who move there permanently, I mean people who spend time there for work or study. Heathrow is one of the busiest airports on the world. Countries like Finland, Chile, etc don't have that. They don't have huge numbers of people coming and going.
Third, London is a vast international city. It's one of the most famous cities in the world, up there with Paris and Rome and New York. Many countries don't have that either. Like Paris or Berlin, London draws in writers, artists, bohemians, etc...people who find small towns and villages stultifying. I met countless foreigners in London who moved there because they were musicians or painters or poets and felt suffocated in their home country. I know a Spanish musician who grew up in a small village and moved to London to pursue her music career and meet "more arty, interesting people". You can tell the difference between people who've grown up in countries with a huge capital city and those that haven't.
Fourth, the British are big travellers. Huge numbers of Americans, in contrast, don't even own a passport.
Finally, the UK contains two of the most famous universities in the world – Oxford and Cambridge. Both draw in students from all over the world, especially the English-speaking world. Many Americans, Canadians and Aussies have a romantic attachment to Oxford, and associate it with people like Oscar Wilde or novels like Brideshead Revisited. In fact, many intellectuals from the English-speaking world are drawn to the UK by its history and culture. They grow up reading the canon of British literature – Chaucer, Shakespeare, Milton, Blake, Wordsworth, Jane Austen, Dickens, etc – and long to make a pilgrimage to the Yorkshire moors to see where the Brontes lived, or to Bath for the Jane Austen festival, or to see where Charles Darwin lived, where Oscar Wilde studied, where Isaac Newton made his discoveries, or whatever. The UK is incredibly lucky in that respect. It has drawn in many brilliant intellectuals from the English-speaking world, from Henry James, Oscar Wilde and T S Eliot to Bill Bryson and Clive James.
The UK is exceptionally non-insular.