The Battle of Penn Hill was fought in AD 665 between a British and an Anglo-Saxon coalition at Penn Hill in northern Dumnonia, the precise location of which is unknown. It resulted in a decisive British victory, and reversed the Saxon expansion westwards across Albion.
Germanic migrants, mainly from the Angle, Saxon and Jutish tribes, began arriving on the east coast of Albion in the mid-5th century, during the chaos following the Roman withdrawal from the provinces of Britannia. They quickly penetrated into the interior of the island, setting up their own kingdoms and assimilating the local Britons to a Germanic way of life. Despite an early setback at the Battle of Baddon, they soon resumed their conquest of the isle, eventually reaching the Mor Hafren in 577.
This put the Britons under extra pressure, as their territories were now divided in two by the sea. In 661 therefore Donyarth ap Culmin, King of Dumnonia, formed an alliance with Visigothic Spain and its client states in Armorica and Lyonesse. Spain sent Dumnonia gold, ships and spearmen, which proved crucial in the conflicts with the Saxons.
In 665 the Anglo-Saxon states of southern Albion formed a coalition to destroy Dumnonia once and for all, since it, along with Powys, was one of only two obstacles to complete Saxon dominion over the island. Urgent negotiations with Powys, and other British kingdoms north of the Mor Hafren, persuaded them to assist the Dumnonians, and the Spanish ships allowed their spearmen to cross the sea to intercept the invading Saxon army.
The kings of Wessex, Mercia and Kent were all killed in the battle, as were hundreds of other thegns and ealdormen. The military might of Wessex and Mercia was wiped out, and the other Saxon kingdoms were in no position to make any more offensive moves.
Exactly what happened next is unclear, but it's known that Dumnonia and Powys both expanded east into former Mercian and West Saxon lands. It's also around this time that the kingdom of Gadwfellwn was founded on former Catuvellaunian territories, which were presumably part of the land reclaimed from the Saxons.
It's also known that Donyarth ap Culmin occupied London for a short period in 668, because he left an inscription above the Ludgate. Possibly he had ambitions to conquer the remaining Saxons, but they were never realised - perhaps because of a new influx of migrants from East Friesland that began around this time.
The only Germanic kingdom in Albion that remained strong was Northumbria, which had not fought at Penn Hill. Its warbands were soon swelled by Angle and Saxon refugees from the south, discourging for the time being any British attempts to destroy Northumbria as well.