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You can make a Subject Access Request https://www.gov.uk/government/publications/requests-for-personal-data-uk-visas-and-immigration/request-personal-information-held-by-uk-visas-and-immigration online or by email. You’ll need to provide evidence to confirm your identity.
You can ask for Basic, Specific, or Detailed information, depending on what you need. You should clearly describe what documents you need, for example a particular decision letter.