It should be noted that the people who live in the British Isles do not use the term, but refer to Scottish English, Welsh English or Irish English (never English English!), or dialects thereof. This article deals with British English in the stricter sense.
The various English dialects differ in the words they have borrowed from other languages. The Scottish and Northern dialects include many words originally borrowed from Old Norse and a few borrowed from Scots Gaelic. Hiberno-English includes words derived from Irish Gaelic.
There are thus many differences between the various English dialects. These can be a major impediment to understanding among the older dialects. However, modern communications and mass media have reduced these differences significantly. In addition, speakers of very different dialects may modify their speech, and particularly vocabulary, towards Standard English.
The accent known to many people outside the United Kingdom as British English is Received Pronunciation, which is defined as the educated spoken English of southeastern England. Earlier it was held as better than other accents and referred to as the King's (or Queen's) English, or even "BBC English". Originally this was the form of English used by radio and television. However, for several decades other accents have been accepted and are frequently heard, although stereotypes about the BBC persist. English spoken with a mild Scottish accent has a reputation for being especially easy to understand.
Even in the south east there are significantly different accents. The local inner east London accent called Cockney is strikingly different from Received Pronunciation and can be difficult for outsiders to understand.
There is a new form of accent called Estuary English that has been gaining prominence in recent decades: it is has some features of Received Pronunciation and some of Cockney. In London itself, the broad local accent is still changing, partly influenced by Caribbean speech. Londoners speak with a mixture of these accents, depending on class, age, upbringing, and so on.
Outside the south east there are, in England alone, at least seven families of accents easily distinguished by natives: