The history of any language can be divided into external and internal history. The former aspect concerns the political and social developments in the community speaking the language while the latter involves the changes which take place over time within the language itself. Needless to say these two aspects are connected to each other but it is a one-way street: the external history can affect the internal one but not vice versa. For instance the rise of bilingualism between the Scandinavians and the English in the north of the country in the 9th and 10th century had repercussions for the structure of English. However, one cannot say that an internal change such as the Great Vowel Shift in any way influenced external developments in England.
What’s in a name?
A variety of names have been used to refer to the country and language of interest here. The word Britannia is a Latin form which refers to the country where the Pritani lived and is derived from Greek Brettanoi which is what Ptolemy called the inhabitants of the island. Anglia is also a Latin reference to the land of the English.
The main name is, of course, English (Old English englisc) which refers to the language of the Germanic tribe from the area of Angeln in present-day Schleswig-Holstein. The word originally had an /e/ as first sound which was an umlauted form of the /a/ of the tribe’s name, the Angeln (Old English Engle). The second term is Saxon which derives from the Saxons, again a Germanic tribe from the North Sea area which came to Britain as of the 5th century AD; the label Saxon fell into disuse fairly quickly.
The study of historical forms of English has had various names throughout its history. The modern labels are Old, Middle and Early Modern English. In the past decade or so it has become common to distinguish a Late Modern Period from 1800 onwards. In the nineteenth century when Old English studies began it was common to call the language of the earliest period Anglo-Saxon. One finds this occasionally nowadays, e.g. in later editions of earlier books on the subject, such as those by Henry Sweet.