Team Sieni:Interesting to consider who the indigenous people of the British Isles might be. We've had Celts, Picts, Gaels, Romans, Danes and miscellaneous Vikings, Angles, Saxons, Jutes, Normans (basically Viking via France) ... (And that's just off the top of my relatively ignorant head)
You listed them in the right order. I don't remember where I read this definition, but it made sense: Indigenous People are people who never replaced other people but were the very first humans who came to an area and stayed there for good, so all the Picts, Gaels, Romans, Danes and miscellaneous Vikings, Angles, Saxons, Jutes and Normans are not indigenous, because somebody was already there when they came - the Celts. But the Celts are indeed one of the three commonly acknowledged indigenous nations of Western Europe - the other two are the Basks in northern Spain and the Saami in Lapland.
However, even this definition is somewhat arbitrary: European nations that replaced other European nations (like the Saxons driving the Celts out of most of Britain) are not considered indigenous, while Native American Nations that replaced other Native American Nations in pre-Columbian times (Like the Navajo taking over the land of the Anasazi) are still indigenous.
If you really want to pursue this any further and turn it into a category (and I think you should), here is a good Wikipedia article on the subject: http://en.wikipedia.org/wiki/Indigenous_peoples