Southern Britain was German in pre-Roman times ?

[caiusbeerquitius]

That wouldn´t surprise me at all. It´s only Ceasar who says that on the right side of the Rhine there are only Germans and on the left side only Gauls. All archaeological sources clearly contradict that statement...

That's right but it's modern interpretations that Gaul = gallic speaking and Germania = germanic speaking which are wrong.

I wasn´t talking about languages at all.

Caesar writes in terms of geographic areas, not linguistic areas. He writes that three languages are spoken in Gaul. We shouldn't conclude that everyone in Germania spoke a german language.

I didn´t conclude this. For comparison Caesar:

I. All Gaul is divided into three parts, one of which the Belgae inhabit, the Aquitani another, those who in their own language are called Celts, in our Gauls, the third. All these differ from each other in language, customs and laws. The river Garonne separates the Gauls from the Aquitani; the Marne and the Seine separate them from the Belgae. Of all these, the Belgae are the bravest, because they are furthest from the civilization and refinement of [our] Province, and merchants least frequently resort to them, and import those things which tend to effeminate the mind; and they are the nearest to the Germans, who dwell beyond the Rhine , with whom they are continually waging war

As far as I can see, Caesar says that all the tribes he is talking about differ in culture, and then he names where the geographical limits of their territories are. The Germans dwell beyond the Rhine.
When reading this it is also useful to remember when Caesar wrote this, and what his political goals were at that time.

It might be interesting to know what these "Germans" spoke, however. If you suggest they didn´t speak Germanic languages, what would you suggest instead?