I can offer you a general overview from a linguistic perspective, although I am, admittedly, not an expert on the history of English or Great Britain in particular.

All languages change over time, and it's important to remember that in the British Isles, English has had over 1500 years to develop since the Anglo-Saxons' arrival. If you look just at the United States, the East Coast has a deeper division into various accents/dialects than the whole western half of the US,.
Barring modern influences like geographical and social mobility¹, the establishment of a standard language in a nation state and general education² or (audio-visual) mass-media, it's simply not possible to sustain a uniform language over a large geographical area. Over time, a language introduced into a larger area will start to break up as every village develops its peculiarities, first into a dialect continuum and, ultimately, into different languages.
Overall, the diversity of English accents/dialects is something that is to be expected.

¹ - An agricultural society, where a small area can sustain a lot of people, tends towards more diversity in a smaller area as people stick around a place. Pastoral or (semi-)nomadic societies can be spread over a larger area (and so their languages on a map), but in turn will support fewer people.
² - For comparison, Continental West Germanic formed a dialect continuum at least until the late 19th century, although it has been breaking up as standard languages - German and Dutch, to be exact - have slowly replaced traditional dialects.