The gallery is full of snakes that twist and slither in hypnotising coils of green and gold. It is hard to stop looking. That fascination leads you into a world that gradually ensnares the imagination. To spend time in the British Library’s blockbuster exhibition about the Anglo-Saxon world is to discover a culture of barbaric splendour and fierce vision, where the real and supernatural entwine.

The snakes are cast in gold and painted in books. You see them on a belt buckle from Sutton Hoo and the abstracted illuminations of Northumbrian gospels. The Germanic people who invaded Britain after the Romans left in AD410, pushing the native Celts into what became Wales, brought these swarming serpentine images from deep in the mists of Eurasian prehistory.

An early text of Beowulf. Photograph: British Library

These snakes are also a troubling metaphor for the madness that swarms Britain’s sense of itself today. The oldest and most enduring of our myths of nationhood is that, in 1066, an “English” nation of plucky Anglo-Saxons was conquered by Normans, who brought French ways such as feudalism with them. This legend of the Anglo-Saxons has been told by left as well as right: the rule of aristocracy was denounced by 19th-century radicals as the “Norman yoke”.

Anglo-Saxon Kingdoms reveals this mythic foundation of nationhood to be as slippery as the eels that were used as currency in dark ages Ely. This vast and engrossing survey of Saxon art and manuscripts reveals a place that was not very English at all. One minute it looks Scandinavian, the next Celtic – but mostly it seems, well, European.

One manuscript in the exhibition is the earliest surviving text of the poem Beowulf. The hero of this great Anglo-Saxon literary work is Scandinavian. The deeper you follow the Anglo-Saxons into their strange realm, the harder it becomes to separate their culture from the rest of Europe.

When the Angles and Saxons first came to Britain, they brought a pantheon of gods they shared with Vikings and fellow Germans. There’s a painting here of Woden, king of the gods – the German Wotan, the Viking Odin. When they started to become Christians, that added another layer of pan-European culture. One of the revelations of this exhibition is how plugged into an emerging Catholic Europe they were. One of the most spectacular loans is the Codex Amiatinus, the oldest surviving complete Latin Bible. It is from the Laurentian Library, in Florence. For centuries the book was passed off as an Italian creation, but a scrubbed-out inscription proves it was created in north-east England and sent as a gift to the pope in 716.

Spong Man, early or mid-fifth century, found on Spong Hill in Norfolk. Photograph: Alicia Canter/The Guardian

It contains a lifelike painting of a scholar that is visibly influenced by ancient Roman art – not a snake in sight. For this show reveals untold sophistication in Anglo-Saxon art. It is clear that when scribes painted in a “barbarian” style, it was a choice. If needed, they could give you classical Roman art instead. They could consult a sixth-century manuscript known as the St Augustine Gospels, which came to Britain with a Christian mission in 597 or 601 and whose illustrations are poised like Renaissance paintings, with classical columns and arches.

Classical learning was so much a part of Anglo-Saxon high society that when Viking raids devastated Britain from the eighth century, one of the side effects was said to be a decline of Latin. King Alfred the Great was so worried by it that he personally translated Pope Gregory the Great’s Regula Pastoralis into Old English so at least his people could get the latest European ideas in translation.

To get young Saxons reading his work he even offered a free gift: 60 handmade copies came with a precious jewelled reading pointer. The Alfred Jewel, lent by the Ashmolean and one of Britain’s most revered national treasures, has a socket thought to have held one of the ivory pointers. Found near Alfred’s Somerset fortress and inscribed “AELFRED MEC HEHT GEWYRCAN” – “Alfred ordered me to be made” – it is a document not only of royal history but of literacy.

Document of literacy … the Alfred Jewel. Photograph: Ashmolean Museum/University of Oxford/British Library

Any exhibition at the British Library is bound to go heavy on the written word. The manuscripts here show an incredible mental transformation. In their complex interplay of word and image we can see a savage, warlike and illiterate people become a European civilisation of the book. The greatest Anglo-Saxon intellectual, the Venerable Bede, points out in one manuscript that the Earth is undoubtedly a sphere, “like a ball”.

It is facile and, as we are learning, dangerous to draw falsely clever comparisons between the inhabitants of these islands more than 1,000 years ago and our dilemmas today. Let’s leave cheap misapplications of history to the Old Etonians. But they might take note that contrary to national myth, the Norman conquest in 1066 was not an alien European incursion into some primal England. The Englishness forged by the Anglo-Saxons, was real, and already defining itself against Celtic Wales and Scotland – there are coins here of Offa, who built a wall against the Welsh. However, this Englishness was sophisticated and consciously European.

It was also remote and mysterious. The Alfred Jewel may be a relic of reading. Yet its socket that once held a book pointer is a golden dragon-head, monstrous and fierce, its surface a mystical tangle of veins and eels. We should look less for national origins in this remote age and celebrate instead the otherness of its brilliant creativity.