Mind your Ps and Qs summary
If historians are not prepared to label the Normans as Romans, or the Anglo-Saxons and Vikings as Germans, then they should be just as unwilling to label Britons as Celts.
From the half a millennium before the birth of Christ, history gives us the Belgae and the Celtae, linguistics gives us P-Celtic and Q-Celtic, and archaeology gives us a large zone with La Tène material and another large zone without it. From all points of view, the historical, the linguistic and the archaeological, we have (at least) two distinct groups, and not one enormous Celtic empire.
Apart from the Picts, who will be mentioned later, the island of Britain has been occupied or conquered by six major groups; Britons, Romans, Anglo-Saxons, Gaels, Vikings and Normans. Of these, the languages of the Anglo-Saxons and Vikings are thought to derive from a common Germanic language, the Romans and the Normans are both known to have spoken variations of Latin, and the Britons and the Gaels, in an earlier period, are believed to have a common ancestor called Celtic. The last two, despite having the most tenuous link of them all, are the only ones that ever get lumped together under a single name.
If historians are not prepared to label the Normans as Romans, or the Anglo-Saxons and Vikings as Germans, then they should be just as unwilling to label Britons as Celts. For all these reasons, the term Celtic will not be used to describe the Britons in this account.