“I’m glad I live on the west side of the River Tamar,” said Rob Tremain emphatically as we discussed Cornish history and politics in the shadow of Launceston Castle. “Driving back home over the Tamar, my dad would always wind down the windows and say: ‘We can breathe again; we’re in Cornwall!’.”

The River Tamar, which forms one of the most ancient borders in Europe, is central to Cornish history and identity, marking a 1,000-year-old divide between Celtic Kernow (Cornwall) and Anglo-Saxon England. Ever since Athelstan, an Anglo-Saxon king, pushed the last native Britons over the Tamar in 936 AD, distinct identities have formed to the east and the west.

Starting as a seemingly insignificant trickle in a muddy field a few miles from the Bristol Channel, the river widens into a daunting natural barrier on its meandering journey 61 miles south to Plymouth Sound. Bounded on all other sides by ocean, I could see how Cornwall’s island-like geography has shaped the peninsula’s history as I plotted my journey into this Celtic borderland to research the often-overlooked story of Cornish culture, history and identity.

Located near the Devon border, the Cornish town of Launceston sits in the midst of this once-bloody borderland and is the first Cornish town that travellers reach when driving along the A30 from Devon. Crossing the river at Polson Bridge – the historical gateway into Cornwall – the green, black and white flag of Devon changed to the black and white of Cornwall as a road sign welcomed me, in both English and Cornish, to Kernow.

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Tremain has been Launceston’s town crier for 43 years. Wearing a Cornish tartan face mask and carrying a tote bag emblazoned with Cornwall’s flag, he explained how centuries of failed rebellions and bloody excursions over the Tamar ultimately suppressed the Cornish language and culture – until the recent Celtic revivalist movement looking to stake Kernow’s claim as the UK’s forgotten fifth nation – but the border has always been a constant fixture.

“This isn’t England, you see,” Tremain said dramatically as we walked under a medieval stone archway into the courtyard of Launceston’s castle. “We’re proud to be British, but we’re not English. We’re Cornish.”

As he guided me through Cornwall’s old county capital, Tremain explained that while the rest of the UK often sees Cornwall as “just another English county”, Cornwall is technically a Duchy. Alongside the Cornish language, it’s a curious historical quirk that’s used to promote the idea of a Cornish “nation” that’s distinct from England.

We’re proud to be British, but we’re not English. We’re Cornish.

Duchies were semi-independent medieval fiefdoms ruled over by dukes and duchesses rather than directly subject to the laws and taxes of English kings and queens. Historically, the Duke of Cornwall collected tithes and royalties from Cornish subjects at Polson Bridge, a tradition dating back to 1337 when the Duchy of Cornwall was first established.

To this day, the Duchy and the vast Cornish estates and revenues that go with it are automatically conferred upon the eldest son of the reigning monarch. Prince Charles, the current Duke of Cornwall, was invested with the title in 1973 at Launceston Castle, and Tremain explained that the peculiar feudal dues he was presented with during the ceremony included: “A grey riding coat, an ashwood bow, a big bundle of firewood and a pint of cumin.”

Leaving Launceston, I crossed back east into Devon before heading south. The Tamar is a formidable barrier, but history on either side of the river isn’t as black and white as the Cornish flag. During the medieval period, the entire south-west region was a land simmering with discontent against distant monarchs; quite often, the Cornish would find comrades-in-arms amongst the equally disgruntled Devonians.

Passing through the Anglo-Saxon village of Lifton, I spotted The Arundell, a pub named for an old family of landed Cornish gentry. Humphrey Arundell had led a Cornish army over the Tamar in 1549 to join the Prayer Book Rebellion, a violent uprising protesting the imposition of a new Protestant English-language prayer book that had begun in the Devon village of Sampford Courtenay.

Although the Cornish (and sometimes the Devonians) rebelled many times, the Prayer Book Rebellion was a turning point for Cornwall’s national identity. Until the 16th Century, travellers crossing the Tamar from Devon would have heard much more Cornish spoken than English. But defeat in the Prayer Book Rebellion sounded the death knell for Cornwall’s ancient Celtic language. The Cornish language withered away as English was enforced in churches, and there would be a wait of almost four centuries before the Cornish would see its modern resurgence,

The winding country roads of Cornwall’s borderland took me backwards and forwards over the Tamar until I reached Gunnislake – or Dowrgonna, as the welcome sign announced in Cornish – where I left my car at the station and walked down to the river to join the Tamar Discovery Trail, a 35-mile long hiking route that runs the length of the River Tamar through the Tamar Valley Area of Outstanding Natural Beauty (AONB) and offers hikers the chance to explore Britain’s forgotten borderland.

The Cornish and Devonians may have joined forces when it suited them, but as Charlotte Dancer, information and communications officer for the cross-border Tamar Valley AONB, explained, the mighty Tamar continues to both unite and divide those on either bank.

“One of the major differences between the two sides is how we eat our scones,” she said lightheartedly. “At one of our AONB conferences, we had plates of scones that had cream on first, then jam – for the Devon residents – and other plates of scones that had jam first then cream for the Cornish residents.”

Culinary differences aren’t the only cultural identifiers marking the Cornish from the English (although don’t forget Cornish pasties, of course). Differences in history and culture can be observed in place names and language, sites of Cornish rebellion and even the layout of villages (Anglo Saxon villages always had a village green, while Cornish villages rarely had them). Karin Easton, president of the Federation of Old Cornwall Societies (Kernow Goth, in Cornish), explained how Cornish culture today encompasses everything from “Cornish pub singing, with altos and sopranos”, to Celtic-inspired Cornish dancing, Cornish brass bands and the “national” sports of Cornish hurling and Cornish wrestling.

I hiked a tough five miles over undulating terrain along the Tamar Discovery Trail until I reached the Cornish village of Calstock, where an impressive 120ft viaduct spans the river. The trail continues on the other side of the Tamar into Devon, but, because the only way to cross the river here is by train (and the next departure wasn’t for another two hours), I decided to settle in for a pint of Doombar Ale at the Tamar Inn.

Calstock viaduct is the last crossing for 20 miles until the river reaches the Tamar Bridge outside Plymouth, before emptying into Plymouth Sound. It was railways, viaducts and bridges like these that opened an isolated Cornwall up to the rest of Britain from the mid-19th Century onwards, bringing mass tourism with them over the Tamar.

Easton explained how, in many ways, it was tourism that helped save Cornwall’s identity when Celtic-ness had all but disappeared after centuries of cultural suppression and Cornish emigration. “From the 1840s onwards, so many Cornish men had to go abroad to find work,” said Easton. “There was a strong feeling to grasp Cornish culture before it disappeared entirely, and this became linked to the idea of promoting tourism to Cornwall, particularly in coastal towns.”

Marketed to incoming British tourists as “the wild land to the west” and the mythical home of King Arthur, a Cornish revival picked up speed as interest in Cornwall’s Celtic past – a past distinct from Anglo-Saxon history – swept through the region from the mid-19th Century. The black and white flag of St Piran, Cornwall’s patron saint, was adopted as a “national” symbol, while the Cornish language was resurrected from the dead, pieced together from fragments of old Cornish texts and dialect, and with a little help from the closely related living languages of Breton and Welsh.

It’s hard for people who come down here to understand the strength of feeling that people have about Cornwall

“The language is still evolving today,” said Easton. “A few people are even brought up bilingually. Not many, but it’s growing. There are Cornish street signs; the park-and-ride bus has announcements in Cornish; schools teach it; and you can learn it online. It’s all around, much more than it ever used to be.”

Easton also explained how tourism has created some of the biggest modern divides between Cornwall and England. The tourism industry is reliant on local workers but Easton continually sees those same Cornish locals priced out of the housing market by holiday-home buyers who live east of the Tamar. The economic difficulties faced by many Cornish in their ancient homeland has fuelled the Cornish revival, which as Easton told me, has led to the election of a number of Cornish councillors representing Mebyon Kernow (the Sons of Kernow), a political group campaigning for a devolved Cornwall with its own parliament – much like Wales, Scotland and Northern Ireland have.

“It’s hard for people who come down here to understand the strength of feeling that people have about Cornwall,” Easton told me. “Some never get it, but those that do, well, they start to identify strongly as Cornish themselves.”

Hidden Britain is a BBC Travel series that uncovers the most wonderful and curious of what Britain has to offer, by exploring quirky customs, feasting on unusual foods and unearthing mysteries from the past and present.

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