Was there actually debate amongst the Norse about the existence or location of Britain?

Ancient norse-peoples definitely did know about Britain.

After-all, it was the early vikings in 367ad which had a great deal to do with the collapse of the British based Roman empire, when early vikings, fought alongside Scots/Caledonians and Irish/Hibernians in order to overthrow the British based Roman empire.

The initial overthrow was successful, however the Romans did reconquer their country, (well, England, they did not reconquer Wales) for a while. But the raids continued, and the Romans were forced to evacuate in 410ad.

The Romans were forced to abandon Wales in 383ad, due to Irish raids.

The great conspiracy

The great conspiracy of 367ad

The Great Conspiracy was a year-long state of war and disorder that occurred in Roman Britain near the end of the Roman rule of the island. The historian Ammianus Marcellinus described it as a barbarica conspiratio that capitalized on a depleted military force in the province brought about by Magnentius' losses at the Battle of Mursa Major after his unsuccessful bid to become emperor.

The conspiracy

The conspiracy

In the winter of 367, the Roman garrison on Hadrian's Wall rebelled, and allowed Picts from Caledonia to enter Britannia. Simultaneously, Attacotti, the Scotti from Hibernia, and Saxons from Germania landed in what might have been coordinated and pre-arranged[citation needed] waves on the island's mid-western and southeastern borders, respectively. Franks and Saxons also landed in northern Gaul. These warbands managed to overwhelm nearly all of the loyal Roman outposts and settlements. The entire western and northern areas of Britannia were overwhelmed, the cities sacked and the civilian Romano-British murdered, raped, or enslaved.

Arrival of Theodosus

Roman response

Once the troops landed, Theodosius marched with them to Londinium which he made his base. There he began to deal with the invaders: There he divided his troops into many parts and attacked the predatory bands of the enemy, which were ranging about and were laden with heavy packs; quickly routing those who were driving along prisoners and cattle, he wrested from them the booty which the wretched tribute-paying people had lost. And when all this had been restored to them, except for a small part which was allotted to the wearied soldiers, he entered the city, which had previously been plunged into the greatest difficulties, but had been restored more quickly than rescue could have been expected, rejoicing and as if celebrating an ovation

Political effects

Political effects

Theodosius returned to Rome a hero, and was made senior military advisor to Valentinian I, replacing Jovinus. A decade later, his son became emperor. The Romans were able to end much of the chaos, though raids by all of the people listed above did continue.

Now Saxon is a Roman word, which pretty much means Viking.

Viking

Saxon, similar to Viking

In the late Roman Empire, the name was used to refer to Germanic coastal raiders, and also as a word something like the later "Viking".3

You see, Saxons are Germanic, and all Germanics derive from Scandinavia

English language derives in Iron age scandinavia

Germanic language

The most widely spoken Germanic language, English, is the world's most widely spoken language with an estimated 2 billion speakers. All Germanic languages are derived from Proto-Germanic, spoken in Iron Age Scandinavia.

Romans abandon Wales

Wales in the Roman era

The history of Wales in the Roman era began in 48 AD with a military invasion by the imperial governor of Roman Britain. The conquest would be completed by 78, and Roman rule would endure until the region was abandoned in AD 383.

Irish settlement

Irish settlement

By the middle of the 4th century the Roman presence in Britain was no longer vigorous. Once-unfortified towns were now being surrounded by defensive walls, including both Carmarthen and Caerwent.[30] Political control finally collapsed and a number of alien tribes then took advantage of the situation, raiding widely throughout the island, joined by Roman soldiers who had deserted and by elements of the native Britons themselves.[31] Order was restored in 369, but Roman Britain would not recover.

Therefore, to say that the Norse-people, did not know about Britain, is simply preposterous, and it deflects from the true history of how Romano Britain fell, and Brittany was created.

Brittany

Brittany

Brittany (/ˈbrɪtəni/; French: Bretagne [bʁətaɲ] (About this soundlisten); Breton: Breizh, pronounced [bʁɛjs] or [bʁɛx];1 Gallo: Bertaèyn [bəʁtaɛɲ]) is a cultural region in the west of France, covering the western part of what was known as Armorica during the period of Roman occupation. It became an independent kingdom and then a duchy before being united with the Kingdom of France in 1532 as a province governed as a separate nation under the crown.

Etymology

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The word Brittany, along with its French, Breton and Gallo equivalents Bretagne, Breizh and Bertaèyn, derive from the Latin Britannia, which means "Britons' land". This word had been used by the Romans since the 1st century to refer to Great Britain, and more specifically the Roman province of Britain.

Quite simply, Brittany was created when Romano-Brits were forced to flee the southern parts of Britain in order to continue living in a Romanised society, after the Romans were partially forced to leave britain due to the raids from several Germanic forces, which included Norse people. (Well, Germanics "are" Norse-people).

Who were the most famous of the Germanic tribes that invaded and settled in Post Roman Britain and imposed their culture on the Britons and turned Britain into the England that the vikings (not Vikings) raided? The Angles and Saxons and Jutes, oh my!

Saxons was a very generic term for Northern German peoples, but any true Saxons invading Britain would have come from Saxony in northern Germany, Angles would have come from the Anglia peninsula and other parts of south Jutland on the modern Danish-German border, and Jutes came from northern Jutland in Denmark.

And there is evidence for cultural connections between the Anglo Saxons in England and other regions in Scandinavia such as Sweden. So apparently travel between the British Isles and Scandinavia, sometimes on a large scale, was not unheard of during the centuries before the viking (not Viking) raids began.

So I don't know what changed to start spectacular and recorded viking (not Viking) raids on the British Isles and western Europe in the 790s. Possibly Scandinavian traders in the British Islands noted that various places there were at the same latitudes as various places in Scandinavia, and thus that it was possible to sail directly across the North Sea at a particular latitude instead of going the long way around the shores of the North Sea. And that might have got viking (not Viking) raiders interested in targeting the British Isles.