No two speakers of English speak the language identically. Every English-speaking country, and practically every region within those countries, has a distinctive accent. It is also true that meanings of words and grammatical forms can vary from one English-speaking country to another, but this entry is concerned solely with the standard pronunciation of British English. This is what linguists call 'Received Pronunciation' or 'RP'. RP refers exclusively to pronunciation, though it can be seen as analogous to Standard English ('SE').
Where Did it Come From?
Modern RP can be described as 'the speech of educated people living in London and the southeast of England and of other people elsewhere who speak in this way'. It developed from what was essentially a regional accent that acquired a unique level of prestige among the ruling and privileged classes in 19th Century Britain, particularly in London. No specific authority gave the accent that would become RP its special status. It just seems to have developed as a consequence of its geographical position surrounding the capital, and thus to the weighty affairs of the rich and powerful.
It may also be said that the development of the Public School system1 in the 19th Century (Winchester, Eton, Harrow, Rugby, and so on) and the ancient universities (Oxford and Cambridge) played a part in establishing the RP accent as the 'standard' voice of the English Gentleman. The result was the marked (and today somewhat ridiculed) accent sometimes called 'ultra-RP', now most often associated with the upper classes and those who have been through the Public School system. This 'marked' RP can be identified as the starchy, standardised pronunciation used by the BBC radio announcers during the 1930s and '40s, which became known as 'BBC English'.
This social aspect of the origins of RP is still nominally acknowledged when RP is called the 'prestige' accent of British English. It can be said, however, that the social distinctions and privileges formerly associated with the accent of 'The Establishment' are becoming less closely tied to 'modern' RP. In fact, RP as it is understood today is less marked than the RP of the early 19th Century.
The Times They Are a'Changin'
Interestingly, in recent times the prestige associated with RP has fallen away. Today 'marked RP' is spoken only by members of the royal family and others from the upper classes. It is considered over-the-top by most people and a strong 'posh' accent is often described as 'plummy' and attracts ridicule. In order to communicate effectively and come across as 'normal', many former public school attendants retain some kind of regional accent or prefer to speak a (albeit usually mild) form of Estuary English. Will Estuary English become the new RP? That's another question.
Today, RP is used by linguists as a 'standard' pronunciation of British English for study purposes and also by teachers of English as a Foreign Language so schools can teach a standard form of spoken British English. When learning a foreign language, it is very unsettling to think of regional variation! RP is seen as the form of English pronunciation most widely recognised and understood within the British national community. Very few British people speak with a strict RP accent but most recognise and understand it. This makes it the most suitable form of the spoken language to teach to people learning English as a foreign language.
This is not to say that alternative pronunciations are to be regarded as wrong or invalid as such. Regional accents are well preserved in many parts of Britain despite the levelling influences of film, television, and radio. It does mean that linguists have established a convention by which a standard pronunciation of British English can be described, written (ie, phonetically transcribed), and read. Modern linguists tend to prefer to take a descriptive rather than prescriptive approach to studying language, and as a result no-one is likely to try actively to 'suppress' such regional variations.
So Who Does Speak RP?
It used to be true that most politicians spoke something very close to RP, especially in the debating chamber. This can no longer be considered valid and even in the House of Lords you can hear a great deal of variety in pronunciation. Nonetheless, many politicians, especially in the Conservative party, still do speak with an accent close to RP. Other professions such as barristers or expensive retailers still normally speak with an accent very close to RP.
A good illustration of the drift away from strict RP is one mentioned earlier - the BBC itself! RP used to be so omnipresent on the BBC (especially in pre-TV days) that 'BBC English' became an accepted synonym for RP. Perhaps the term no longer conjures up the same mental 'sound-image' as it once did so universally. 'And now, here is the news at one o'clock.' If nothing else, it is simply no longer the case that most of the voices coming out of our TV or radio sets are speaking RP. Perhaps the last bastions of RP on the BBC are Radio 4 and the BBC TV news, both of which still require maximum clarity and the widest possible range of comprehension.
The context in which RP is probably most widely used is the academic world. Teachers of English as a Foreign Language were mentioned above but the great majority of school teachers within Britain, whether they have regional accents or not, will tend to speak in an accent as close as possible to RP in order to communicate effectively. This is not to say that a regional accent should be a barrier to their ability to teach, but nonetheless RP is considered appropriate in the classroom, particularly in higher education.
University lecturers want to convey new information to a large number of students simultaneously, often involving complex ideas. In such a context, we can see the value of an exceptionally clear and universally understood standard pronunciation. Students need not imitate this accent in order to assimilate the information but the fact that it is expressed in RP may well help them to understand more easily.
Perhaps surprisingly, one group of English speakers who will tend to stay close to RP are very advanced speakers of English as a second language. It is usually quite noticeable because so few native-speakers speak so clearly. People who have learnt English are unlikely to adopt linguistic habits that diverge from RP because they learnt the RP 'standard' pronunciation. Of course, a non-native speaker of English can develop a regional accent, if, for instance, they came to an area with little or no knowledge of English and learnt the language entirely in that region.
Finally, for whatever reason, some people choose to maintain a relatively strict RP accent in life. For some RP may seem the only natural way to speak. This group is becoming smaller as the years go by, though, and many native speakers may associate a strongly RP accent with negative ideas ranging from 'stuffiness' through 'pedantry' up to 'snobbishness' and 'privilege'. For all that, there is something wonderfully 'English' about certain bygone RP speakers such as David Niven, Trevor Howard, Sir Richard Attenborough or even Terry Thomas that will always have an appeal in some quarters of the UK.