[Quoth]

There is no English of mid 5th C.*
Even English of 14th C (Chaucer) is barely readable compared to late 15th C.
The first Old English literature dates from the mid-7th century and British English readers wouldn't be able to read it.

The current edition of the King James Bible isn't the original and a lot of people would have difficulty with 17th C. Shakespeare.

English in the Americas didn't change that much between 16th & 18th C and was very diverse. It changed a lot in 1840s due to Noah Webster's prescriptive Dictionary and his obsession with his spelling rather than actual usage. In contrast the Oxford English Dictionary was not prescriptive and reflects usage.

So USA English is largely a 19th C. invention by one man and British English drifted along with changes and still does.


EDIT
[* Anglo-Saxon in "Angle-Land" (-> England, imagine if Saxon the dominant, SaxLand or Sexland) starts in the 7th C. and drifts. Massive changes after 1066 and the Normans, who didn't speak ordinary French and added "Qu" spelling and "-our" endings etc. Noah Webster deliberately suppressed Norman influence in English and changed US spelling. Amazing Webster didn't change Queen to Kwean etc.]