For much of prehistory this little bit of earth we now call England was part of the land mass of north-western Europe- with people traipsing into it and out of it (between ice ages) across the low-lying area we now call Doggerland. Then sea levels rose, Doggerland was swamped and England became part of a complex of islands. Up until the Roman invasion it was divided into a patchwork of tribal territories- which the Romans united and incorporated into their empire as the Province of Britannia. After the Romans left it was divided up again- into a shifting patchwork of little Kingdoms- mostly ruled by comers-in from Northern Europe. These Kingdoms were united under Alfred the Great and his successors and by the end of the Saxon period something like the modern borders of England (Angle-land) had been established. The Normans invaded in 1066 and incorporated England into an empire based in Northern France. This empire was in a constant state of flux. At its furthest stretch, under the Angevins, it reached from the Grampians to the Pyrenees (I miss the Angevin Empire). Edward I added Wales to the portfolio. Oliver Cromwell, after something like 500 years of imperial adventuring, added Ireland. The last French territory (if you don't count the Channel Islands) was lost under Mary I. Shortly afterwards England began to acquire an overseas Empire which- at its height in the late 19th century- covered a third of the globe. Henry VIII took the country out of Catholic Christendom, and something like our modern sense of it as special and separate ("This precious stone set in a silver sea") came to maturity under his daughter Elizabeth. The border between the kingdoms of England and Scotland had always been porous, with towns like Berwick-on-Tweed regularly changing hands. Under James I the crowns were united, with full political Union following in 1703. Henceforward England was part of a United Kingdom, otherwise known as Great Britain. By the end of the 19th century "English" and British" had become interchangeable terms- with even Scots (Stevenson for instance) happy to rabbit on about their "Englishness". In the 20th century Britain lost its Empire (including Ireland), gained a "commonwealth", ceded foreign policy to the USA, joined the EU and opened its borders to mass immigration At the time of writing the Union between the remaining portions of Great Britain has been weakened by the creation of a Parliament in Scotland and an Assembly in Wales- with the strong possibility that the Scots will vote for complete independence in a year or two.
I'm English, or perhaps I'm British; I'm not entirely sure. And what, anyway, do either of those labels mean?