So not trying to argue, just have a genuine conversation. Talking from an American perspective, British implies the British Isles (place) to me and not the people (who I honestly have no idea when they lived or currently live there??). I'm not familiar with any other name for the isles (again speaking as an American).

Also, I think (or hope) that most people would understand and any research into the Iron Age is only showing a survival bias that may not indicate the whole population in an area. However, I think it's fair to state that if you see a trend across multiple sites in roughly the same time period, it indicates a larger cultural practice in that area because we are only seeing a small amount of surving evidence. For this reason, I think "some" is too broad.