The first would be hunter-gatherers who settled the ice age region some 15 thousand years ago, give or take a whole lot (I'm no expert). Since the Celtic, italic, and germanic people who have since inhabited it were all indo European, and since it is generally acknowledged that Indo-European peoples left their original home in Ukraine some six thousand years ago, it's only safe to say that these ancient people - who likely erected Stonehenge and other monuments as far north as the Orkney islands - were "someone else". I've seen hypotheses ranging from an ancient people who later became the Basque/Etruscans (mostly to explain these languages' presence without clear relatives) to another that postulated Uralic people were once dominant across northern europe and left a noticeable substratum in Germanic languages today. Most likely both of these are wrong and there was just another genetic group of humans, recently out of Africa, who preceded later migrations.