Spread of the London dialect

the Great Vowel Shift.
The Great Vowel Shift was a major change in the pronunciation of the English language that took place in the south of England between 1450 and 1750.[1] The Great Vowel Shift was first studied by Otto Jespersen (1860–1943), a Danish linguist and Anglicist, who coined the term.[2]
The Great Vowel Shift involved six vowels: all were long, stressed monophthongs -- vowels in stressed positions which were pronounced long and had a pure sound. For the non-linguist, what I have just written probably sounds like academic gobbledygook, so let’s look at some specific examples.
The vowel “i” as in “mice” is a high front vowel. In Middle English “mice” would have been pronounced “mees”.
The modern word “mouse” would have been pronounced “moos” in Middle English. It then evolved into “mah-oose” and then finally into the dipthong which we have today in “mouse”.
The vowel “e” as in “feet” is considered a mid-vowel. In Middle English “feet” would have been pronounced “fate”.
Another mid-vowel is “o” as in “do” which would have been pronounced “dough” in Middle English.
The vowel “a” is a low back vowel. The modern word “name” would have been pronounced “nahm” in Middle English.
The long open “o” which was pronounced “aw” became the long “o”. Thus the modern English word “so” would have been pronounced “saw” in Middle English.
Linguists summarize that in the Great Vowel Shift: (1) front vowels were raised and fronted; (2) back vowels were raised and retraced; and (3) high vowels were made into diphthongs.
Linguists have documented the fact that the Great Vowel Shift happened, but the intriguing question is why did it happen? In answering this question, linguists offer two non-exclusive hypotheses.
changes in early modern English.
By the sixteenth century English spelling was becoming increasingly out of step with pronunciation owing mainly to the fact that printing was fixing it in its late Middle English form just when various sound changes were having a far-reaching effect on pronunciation.
Chief among these was the so-called ‘Great Vowel Shift’.

• 
  long i became a diphthong (probably in the sixteenth century pronounced [əi] with a first element like the [ə] of the first syllable in ago)
  
• 
  long e took its place with the value [i:]
  
• 
  long a became a front vowel, more like that of air to begin with, but later [e:].
  

Numerous conditioned changes (i.e. changes in the sound of a vowel or consonant when in the vicinity of another sound) also contributed to the mismatch. When long vowels were shortened in certain positions a given spelling could show either on the one hand a long vowel or diphthong or on the other a short vowel that would normally be spelt another way.
At the start of the sixteenth century the main systematic differences in spelling from present-day English were as follows.
i). u and v were graphic variants of a single letter. The form v was used at the beginning of a word and u in all other positions, irrespective of whether the sound was a vowel or a consonant.
ii). Similarly, j was only an extended form of i. i was generally used for both the vowel and for the consonant sound (as in jam) in most positions in a word: its capital form, which resembles J, was beginning to be used in initial position for the consonant sound.
iii). The final ‘silent’ -e was much more commonly found, not only as a marker of a ‘long’ vowel in the preceding syllable (as in take), but with no phonetic function, and sometimes after an unnecessarily doubled final consonant.
iv). The letter y was commonly used for the vowel i, especially in the vicinity of ranging or ‘minim’ letters such as m, n, and u.
v). Double e (ee) or e..e was used for two different long front vowels
vi). Similarly o (oo) or o..e were often used for two different long back vowels: the ‘close’ vowel of moot and the ‘mid’ vowel of moat, mote. o..e was gradually restricted to the latter and, during the 16th century, oa was introduced on the analogy of ea.