A while ago I discussed a theory that Germanic languages were spoken in Britain long before the arrival of the Anglo-Saxons.
Today I came across an article by Stephen Oppenheimer, author of The Origins of the British: A Genetic Detective Story, in which he argues that many people in what is now England spoke Germanic languages quite a long time before the Anglo-Saxons showed up.
Julius Caesar mentioned that a tribe called the Belgae had settled in parts of soutern Britain before he invaded the country, and that they spoke essentially the same language as their continental cousins. The Belgae are thought to have spoken either a Celtic language or a Germanic one. Oppenheimer thinks they probably spoke a Germanic language, and Caesar implies as much.
Among the evidence for a pre-Roman Germanic-speaking population in Britain, Oppenheimer mentions the near absence of place names of Celtic origin and Celtic inscriptions in most of England, the handful of Celtic words in English. He also uses genetic evidence to demonstrate that there is are significant Scandinavian elements in the genes of people from the east coast of Britain from the Shetlands to East Anglia, and that these elements dates back to Neolithic times. He also cites lexical evidence that suggests that the split between English and the Germanic languages spoken on the continent of Europe goes back a lot further than conventionally thought.