The usual conservative estimate is 3000 years ago. That could be far too conservative, given the age of British settlements. We know that the seas of Northern Europe were highways, not barriers at least 5,000 years ago and probably 8,000. Even St Kilda was occupied 5,000 years ago. That was some feat of navigation.
Myth suggests there was contact between the Mediterranean and the Atlantic communities.
I trust that name invokes some thoughts. Because their scholars, led by Gordon Childe, popularised the Diffusion Theory, with Mesopotamia as an epicentre from which an earthquake of human genius spread its tentacles. But archeologists have found in Sumeria evidence of experimentation with tin and copper, resulting eventually in the correct formula for bronze. So the textbooks tell you that the Sumerians invented bronze. The Sumerians had no tin and no copper, so for them to have invented bronze would have required divine intervention.