The simple answer to how to speak with a British accent naturally is listening practice. Which means lots of listening to natural English speakers who are speaking received pronunciation (sometimes called The Queens English) which is the British accent used in the South of England.
The full article can be found here: https://adeptenglish.com/2019/02/04/british-accent-practice/
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Summary: British Accent Practice
The more you tune your hearing to the British accent the more normal it will sound to you. If you listen to Americans speaking English or Australians, then you will adopt their accents.
Naturally we (Being based in London, UK) would say the British English accent is the easiest to listen to. However, we are bias! If all you want is to speak English then any English-speaking accent will do. We advise you to stick with one though, in the early days we recommend you listen to one speaker's voice in one accent.
When you think about it, that's quite a commitment, you will need to make sure that there will be enough appropriate listening material to study from that one voice.
 


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