The description of standard British English today does not require centring diphthongs. In this it’s very different from the earlier prestige accent Received Pronunciation. RP’s system of centring diphthongs was a transitional phenomenon between the rhoticity which lasted to the 18th century in Southern Britain and the more monophthongal system we hear in the 21st.
The source of the centring diphthongs (diphthongs ending in schwa) was the vocalization of r when not before a vowel, simplistically as follows:The monophthongization of the resultant centring diphthongs proceeded gradually but systematically, spreading first up the back vowels and then up the front vowels. So aə was the first to monophthongize; by 1791, dictionary-maker John Walker reported that in London bar was pronounced baa. In the 19th century oə monophthongized, merging with the THOUGHT vowel, though the centring vowel persisted in some words (e.g. hoarse) into the early 20th century, when RP was first codified.
Next to shift was uə, with an increasing number of words like your, sure and poor joining the THOUGHT set, though alternatively some words like cure and Stuart have become (or remain) sequences of GOOSE + schwa, while the first syllables of words like Euro(pe), tourist and during now exhibit a range of possible monophthongs.
Last to go were the front counterparts. The SQUARE vowel has generally monophthongized to ɛɛ. The first element of NEAR has shifted from KIT to FLEECE, so that NEAR words can be seen as sequences of FLEECE + schwa. Thus beer is identical to be a, as in this punning headline:A dynamic demonstration of the difference between RP’s “centring” nature and the more monophthongal system of today is provided by the optional phenomenon known as smoothing. (I think the term is due to John Wells.)
Contemporary smoothing works very differently from the old smoothing of RP, which had this form:
V ɪ/ʊ + ə → Və
(when a closing diphthong is followed by schwa, optionally omit the second element of the diphthong)
So, in RP, fire faɪə could become faə and power pɑʊə could become pɑə. Thus old-style smoothing had the effect of adding centring diphthongs to RP’s repertoire, even reinstating historically lost ones. Given the relative scarcity of centring diphthongs in the world’s languages, the prevalence of them in RP (ɪə, eə, aə, ɑə, ɔə, ʊə) was one the features that gave the accent its distinctive flavour – a flavour which is perceived as very old-fashioned today. Here from 1948 is George VI (the one portrayed in The King’s Speech) saying “It gives me much pleasure… to decl[eə]… [stammer] the Kingston p[ɑə] station open”:
Contemporary smoothing, by contrast, has this form:
V j/w + ə → VV
(a closing diphthong followed by schwa optionally becomes a long form of the diphthong’s first element)
So unlike old smoothing, which created centring diphthongs, contemporary smoothing produces long monophthongs, e.g.
PRICE ɑj + ə → PALM ɑɑ
FACE ɛj + ə → SQUARE ɛɛ
CHOICE oj + ə → THOUGHT oo
MOUTH aw + ə → lengthened TRAP aa
Here is opera singer Simon Keenlyside smoothing variety from var[ɑjə]ty to var[ɑɑ]ty, which could rhyme with party:
And here is a TV interviewee smoothing liabilities from l[ɑjə]bilities to l[ɑɑ]bilities:
To demonstrate the monophthongal nature of the smoothing, I’ll now insert the speaker’s final fricative after his smoothed ɑɑ, then use a sliver of the fricative to make a t-burst – the result is a pretty natural-sounding last lɑɑst:
(I take this kind of evidence as confirmation that the standard BrE PRICE vowel is ɑj, with a first element identical in quality to PALM/START.)
When the MOUTH diphthong is followed by schwa, smoothing produces a lengthened version of the diphthong’s first element, i.e. TRAP. Here is chef Sam Clark saying sourdough with an unsmoothed MOUTH-schwa sequence in the syllable sour:
He also produces the word in a version with a smoothed first syllable, i.e. containing a monophthongal TRAP quality a:
The first part of this word sounds like a longish pronunciation of the word sad:
When the FACE diphthong is immediately followed by schwa, the sequence is optionally smoothed into a lengthened version of the diphthong’s first element, DRESS, creating the long SQUARE vowel. Here is UCL’s Prof. Mark Miodownik saying thinner layers with a FACE-schwa sequence in layers:
But here, in an atomic layer of graphene, he produces layer smoothed to lɛɛ:
When the GOAT diphthong is immediately followed by schwa, there is no difference between the effects of old smoothing and those of the newer kind. Since GOAT begins with schwa, the result either way is a long schwa, i.e. the NURSE vowel. Here is John Wells saying lowered with such a monophthong:
NEAR words can be thought of as sequences of FLEECE + schwa. FLEECE itself is a narrow diphthong ɪj, so that near in its most careful pronunciation is nɪjə. This may alternatively undergo smoothing, which produces a long version of the first element: ɪɪ. Here is Daniel Radcliffe pronouncing weird monophthongally as wɪɪd:
And here is radio/TV presenter Dominic Byrne saying Here is a lady with hɪɪ:
We can confirm that this is ɪɪ rather than ɪə by removing the beginning of the first word, leaving what sounds like It is a lady with t-glottaling:
Smoothing is particularly common before a following l, via schwa-insertion or “breaking”. So I’ll or aisle may be pronounced ɑjəl, or may be smoothed to ɑɑl, as in this clip of Prince Harry:
Similarly, mail or male may be pronounced mɛjəl, or may be smoothed to mɛɛl. Here is BBC presenter Emily Maitlis saying email with a second syllable which could rhyme with there’ll:
(In fact she says this quite rapidly, so that it could rhyme with tell. This kind of shortening can happen to any long monophthong in running speech.)
When CHOICE + schwa are smoothed, the result is the THOUGHT monophthong (as in the other cases, a lengthened version of the diphthong’s first element). Here is telecoms honcho Richard Hooper saying Royal Mail with Royal smoothed so that it rhymes with all (he also smoothes Mail):
Another difference between the old and the new smoothing is that the latter is less likely to occur in absolute utterance-final position. So fire can be fɑɑ in fire engine, sounding like far engine, but is likely to remain fɑjə when utterance-final. However, we can certainly find instances of contemporary smoothing in final position. Here is Sam Clark saying half an hour with pre-pausal hour clearly smoothed to aa:
And here is BBC presenter Andrew Plant saying utterance-final this year as ðɪs jɪɪ:
Unhelpfully, despite its obsolescence, the centring-diphthong system of old RP is still promulgated in dictionaries and elsewhere. It’s worth pointing out that the less prestigious accents of Southereastern Britain are much more conservative in this respect. I was recently on the London underground, where the official standard BrE voice told us that the next station was Leicester Squ[ɛɛ]. The driver then interrupted to tell us in Popular London English that, as Tottenham Court Road station was closed, passengers for Oxford Street should change at Leicester Squ[eə]. So if you want a relatively low-class accent, by all means follow the dictionary transcriptions.