The manin basis for locating Breguoin at the Roman Fort of High Rochester (Bremenium) is the name. The old Welsh name of Bremenium is Brewyn, and these names are similar enough to possibly the same.
The Roman fort of High Rochester is in Northumbria close to the small town of Rochester. The fort was the most northerly garrisoned by the Roman Army and commanded the river valley below. It is known to have been used during the later sixth century. The fort's position is strategic as well, covering one of the best routes through the hills of this area. Arthur could have fought to halt an enemy advance through these hills.
An alternative site has been proposed, in Herefordshire. This is another Roman fort with a similar name, Bravonium.
The Battles of King Arthur