Germanic-speaking Keltoi

It's entirely possible that the languages that were mutually intelligible across the channel between ancient Belgium and Kent at the time of the invasions of Julius and Claudius Caesar were not what we would call Celtic languages today, but Germanic ones. Indeed, perhaps many of the peoples referred to by ancient Greeks and Romans as Celts were Germanic-speaking, and the eastern half of the island of Britain spoke languages of the Germanic family long before the arrival of the Romans. (This theory isn't very popular, nor is there any way to prove or disprove it conclusively at such remove, but I for one find it very plausible).

http://www.proto-english.org/l4.html