That is true. The Saxons, which eventually united England and subjugated the rest of the UK weren't nice to it's neighbors. But the Saxons were made up of French and German tribes to begin with.
The original inhabitants, the Britons or Celts were pushed into Brittany and absorbed by the Saxons.
Great Britain was such a mess they had to eventually invite the Dutch to invade them and rule them.
Recent geneological tests of the British population have shown that the Celts never went anywhere. No one in modern Britain is more that 30% Anglo-Saxon, the rest being the original Celt. Apparently the invading German tribes were simply aborbed into the much larger Celtic population.
The French were German as well. The word French comes from Franks, one of the invading Germanic tribes. A great many people do not realize that the great "French" ruler Charelmagne and his father Charles Martel, spoke a form of German. Over time the German speaking invaders were absorbed into the Latinized population of what used to be called Gaul and which we now call France.
As for the the Dutch - they never did invade England. The English deposed their king, James II and invited his daughter Mary to take his place. Mary, very convenienty, happened to be married to William of Orange, the ruler of the Netherlands, one of the great generals of the 17th century and a man capable of defending his wife's throne. It was a great coup for William, who now had one of the most powerful nations in Europe to use against his greatest enemy - France.