Note: this RP Accent Breakdown is currently a work-in-progress - bear with us while we research and add to it!
The 'L' Sound
RP, unlike many accents, contains both light and dark L's. The light L is made by contact with the tip of the tongue to the alveolar ridge, as in the RP pronunciation of the word 'light'. The dark L is also made with the tip of the tongue, but more of the body of the tongue also contributes to the shaping of the vowel, creating a darker sound. This is heard in the word 'ball'.
Some accents, such as Welsh, Irish, and Nigerian, lighten all of the L's. Some others, such as American and Scottish, darken them. For RP, the rule is that if the L comes before a vowel sound, it becomes light. If it comes at the end of a word, or before a consonant, it darkens. For instance:
Paul - Dark
Paula - Light
Old - Dark
Olly - Light
Loll - First Light, Second Dark
Lolly - Both Light
The 'R' Sound
RP is a non-rhotic accent. This means that R sounds that come after vowels are often not enunciated. In a rhotic accent, such as West Country, Irish, Welsh or Scottish, all of the R's are pronounced. For instance, in the phrase "are you bored?", RP does not pronounce the 'r' sounds, whereas a rhotic accent would. SoRP leaves out 'r' sounds that come after vowels. This means that the vowel is elongated, and the tongue does less work.
However, 'r' sounds that come before vowel sounds still get pronounced fully. The word 'rear' has one 'r' at the beginning (which is important to differentiate it from 'ear'!) but RP does not pronounce the word at the end.
"Start here" - Both r's out.
"Red Lorry" - Both r's in, as they occur before vowel sounds.
"Great Start" - First 'r' in, second 'r' out.
One final rule - the linking 'r'. If a word ends with an 'r' and the next word starts with a vowel, you then pronounce the 'r'. In "the car is wrecked", you pronounce the 'r' in 'car' because the next word starts with an 'i' sound.
"See you at the bar" - 'r' out.
"See you at the bar in five" - 'r' in, because the next word starts with a vowel.
The 'y' sound
In RP, the 'y' sound, also called the yod, is very important for clarity. In certain words, such as issue and soldier, speakers are tempted to say 'ishyu' and 'solja', creating a slushy centre to the word. For RP, we tighten up this pronunciation with the 'y' sound - making 'ishyu' into 'isyu' and 'solja' into 'soldya'. For the phonetically-minded, the difference is clearer in IPA:
/sɒɫdʒə/ and /sɒɫdjə/
/ɪʃjuː/ and /ɪsjuː/
So the words 'tissue', 'issue' should be a perfect rhyme with 'miss you'.
This is not to say that all 'sh' sounds, such as in 'militia' or 'exposure', should be changed to 'milisya' or 'expozya'. But in certain words, this distinction is very important.
Word Endings: Z's and S's, T's and D's, 'Th' and 'F' sounds
RP is one of the clearest accents in the English language - it originated in the English law courts because speakers had to be heard clearly over long distances. This clarity is reflected in its precision over word endings - every word ending has to be enunciated clearly and precisely. Elision - the running of one word into another - is a big, fat no-no.
Z's and S's are largely the same consonant but for one crucial difference - on the 'z', the voice comes underneath the consonant. On the 's', there is no voice whatsoever. You can put your hand on your larynx to feel the difference in the vibration. When pronouncing 'z' sounds, the z can tend to be de-voiced. For instance, in the words 'owls', 'balls' and even 'words', all of the 'z' sounds on the ends of the words can turn into s's. It can take a little attention and practice to keep the 'z' sounds strong.
Sounds, drowns, bounds, gowns, hounds - all have 'z' sounds on the end, despite being spelled with an 's'.
T's and D's are also pairs, except the D has voice and the T does not. Some accents, such as Essex and some parts of London, can devoice the 'D' and make a 'T' instead. This can be found on 'today', 'tawdry' etc. Other accents, such as some Indian and Middle Eastern accents, discard the 'T' altogether and replace them with D's. 'Today' becomes 'doday' and 'total' becomes 'dodal'.
A lot of accents also remove the 't' from the middle and ends of words, replacing it with a glottal stop. For instance, a cockney speaker will say 'bu'er' rather than 'butter', and 'ou'' rather than 'out'. The words 'potter', 'fight', 'total' and 'port' have the same danger.
Finally, lots of speakers also use a 'v' or a 'f' for a 'th' sound. So 'further for the feather' becomes 'furver for ve feaver' and 'faith' becomes 'faif'. For the 'th' sound, the tongue touches the top teeth - but for the 'f' it is the lower lip that does the work. This means that the tongue needs to be trained to make contact with the teeth effectively instead of using the lips to create the sound.