The mysterious ancestors of the Britons came after the people who built Stonehenge (95% of these Neolithic people were replaced by them), but long before the arrival of the Celts. They brought their metallurgy skills and horses from the Netherlands. This was the single most important migration event in British history, but until recently little was known about it.

Note: As I have said before, one of the things we have to look forward to in the 2020s is how much more we are going to learn about ancient prehistoric migrations due to advances in genetics that are going to begin to lift the veil on the distant past.