England Before Her Monarchs
The Celts migrated to Britain, among other places, at some undetermined time in the BCs. They kept themselves to themselves, and no one outside of Britain knew of its existence until around 200 B.C. The Phoenicians, the world's pre-eminent traders of the time, thanks to their high-tech boats and sailing ability, would go out scouting for new places where no one had gone before. They did this not only for adventure; the hot commodity of 200 B.C. was tin, and supplies dwindled due to the huge demand, forcing traders to look for new sources. When the Phoenicians found two small islands far to the west, they also found two rich sources of tin (and not much else, save sheep and a basically worthless kind of pearl). Naturally, others were curious as to how the Phoenicians came up with all this tin all of a sudden, but the Phoenicians refused to tell, and, true to their standard policy, would destroy their ships and crews if they couldn't shake a pursuer. This time the stakes were much higher than usual, however, and, thanks either to unusually determined pursuit or a Phoenician crew with a fondness for life, the whereabouts of the two islands became generally known.
The larger of the two islands was given the name Brittonum, and the smaller was named first Aire and then Scotland. A number of the inhabitants of Scotland emigrated to the upper half of Brittonum, however, causing that area to be named Scotland and the smaller island renamed Aire, whence Ireland. The new Scots were renowned for their ferocity, but surpassing them even were the natives of the newly named Scotland, the Picts, famous for entering their battles painted blue. The Picts died out mysteriously during the reign of the Scottish king Kenneth Macalpin, in the twelfth century.
Brittonum was eventually divided into various kingdoms, among them Wessex, Northumbria, East Anglia, and Mercia. Each kingdom had its king, and according to tradition, one of those kings was chosen to rule over all the rest as high king, or Bretwalda. This notion was pooh-poohed by scholars for years as a pretty, patriotic fantasy, until the Sutton Hoo excavation in the 1950s of the the burial ship of one of the Bretwalda. How much actual power the Bretwalda had is unknown; scholars now think it was a more or less honorary position, but that may or may not be. In any case, certain kingdoms from time to time gained more than their usual share of power, and in the ninth century, King Egbert of Wessex conquered some of the surrounding kingdoms and (accordng to legend) proudly called the whole, Angle-Land, whence England.