There is little graphic humour about being British. That was clearly less fun to visualise, harder to pin down and to parody than the stereotypes of the four nations. What we all shared was the crown and its ministers, and these the caricatures treat with a savagery that shocked foreign visitors, as they are still astonished by the rough handling by our media of politicians and royals. This wilfully coarse-grained insubordination, mixing insults political and sexual, scatological and religious, was without parallel in Europe, leaving outsiders unsure as to whether this was liberty or decadence. And it is against these same outsiders that Britishness is shown in a positive light, especially when foreigners take the haunting and hateful form of the French.
But there is in fact much more about the British Museum that is British than this collection of visual abuse. Above all, there is the name. Set up by act of parliament in 1753, the museum was the first public institution to be called British. Nobody quite knows why, but it must have been meant to embody the values of the new state created in 1707 and severely tested by the Jacobite rebellion of 1745. So perhaps it was British because it was not to be royal in any sense but to be used as of right by all citizens. What is certain is that it was a new kind of civic venture, a huge collection of books and objects exploring every aspect of history which was to be open to everybody, "native" and "foreign", in the striking words of its first director, and above all to be free of charge.
It was the first national museum in the world but had nothing nationalist in its purpose, and it remains one of the great achievements of the Enlightenment, an enduring statement that the public realm is intellectual and spiritual as well as physical and economic, and excludes nobody. In 18th-century Europe, this was a uniquely British contribution. Indeed, it still is today.
There was, of course, another sense in which it was a British first, for it was one of the earliest physical consequences of a truly global economy. Thanks to the unprecedented reach of British navigation, London in the early 18th century was not just the emporium of the world, it was the first place in which it was possible to assemble artefacts from around the world and allow people to study them. Never before had it been possible to compare the different continents, to consider the world as one.
The museum was British because it was for all who found themselves in Britain who would need to know about the world if they were to flourish in it. Oxford and Cambridge were restricted to members of the Church of England. But the museum was to be British, an open university and library that everyone could use on equal terms. Like the caricatures, this 18th-century fusion of local and global, this need to rethink the world as one, are now again exhilaratingly topical, and particularly British.
More than any other European country, Britain has a world population. The recent museum focus on Bengal, the Middle East or modern Ghana allowed us to tell stories that are as much about Britain as about Kolkata, Baghdad or Accra, not just because of the communities here from those regions but also because many of the artists, especially from the Middle East, could not now work in their country of origin. In Britain, as in 18th-century London, the categories of "native" and "foreign" are increasingly unhelpful in describing what is important about its people. The links they maintain with their countries of origin put Britain again at the centre of a conversation with the whole of humanity.
The museum does not always tell the truths people want to hear. It was set up to challenge the simple labels with which people addressed the world. The ideal of tolerant inquiry it embodied has outlived the 20th century's disastrous fantasy of the nation as a closed cultural community.
The Britishness that allowed four querulous caricatural nations to live together, that conceived a public realm capable of embracing the world, has perhaps still its most important contribution to make. For it offers perhaps the best available route to constructing identities for the new century, through complexity and difference, both requiring tolerance. Dangerous territory, but it is where the British Museum was meant to be. If we are looking now for values that are distinctively British, one of them must surely be that 18th-century ideal: that access to information and knowledge, to the greatest achievements of humanity, must be free to all. Neil MacGregor is director of the British Museum. Who needs to be British?, a Guardian/British Museum forum chaired by Jon Snow, with Lisa Jardine, Andrew Marr, Bob Rae and Ziauddin Sardar, is at the museum on Tuesday at 8pm.
For tickets call 020 7323 8181 or visit www.thebritishmuseum.ac.uk/tickets