The Angl comes from the Angles who settled parts of England shortly after the Romans. It is believed the name refers to the part of Germany they came from Angel or Angeln. Naturally you would then expect the country to be named Angland. However, between the 14th and 18th centuries England experienced The Great Vowel Shift "a" eventually came to be "e". This time period also represented the period when printing began to standardize the English language and spelling. Words in common usage such as England were often spelled how they sounded but words not in common usage such as Angel, as in Anglo Saxon, which would usually be only seen in writing and used by scholars were spelled as they were written.

The reason Anglo- is used is because that's the Latin root, which is the convention. See, for example, Hiberno- for Irish, Gallo- for French, Sino- for Chinese etc.

England used to be called (pre-Norman) variations on Angleland, taking its name from the Angles who had emigrated from Germany along with the Saxons (hence Anglo-Saxon), bringing their Germanic languages with them. Alfred the Great, for example, called his country Angelcynn and his language Englisc.

With the Norman invasion, Norman French rather than Old English came to be the language of court for several hundred years, until Middle English became popularised after the Hundred Years War. By this time, the English language had undergone a process of standardisation, and pronounciation had changed significantly - part of a process known as the Great Vowel Shift.

You might also have noticed the name for England in German is England, which further illustrates the German origin of the name.