How did English invasions affect the older British Celtic language in Britain?

The English invasions, which began in what is now southern England and eventually extended toward Scotland, played a significant role in the fragmentation and decline of the older British Celtic language (also known as Brythonic).1

As these invasions progressed, they contributed to the linguistic partitioning of Britain, disrupting the unity of the British dialects. This pressure led to the displacement of British Celtic in many areas, resulting in its survival only in isolated pockets. For instance, a British dialect known as Cumbric persisted in the western borderlands between England and Scotland until approximately the 10th century, while other forms survived in Wales—where it evolved into modern Welsh—and in Cornwall, where it remained until the end of the 18th century.1

Ultimately, the expansion of English-speaking territories fundamentally altered the linguistic landscape of the island, relegating the once-dominant British Celtic language to the western fringes of Britain.1