Digital Collections of Letters and Manuscripts written by individual Britons (Others w/regional significance are listed in those categories):
George Orwell Archive at UCL, includes manuscripts, letters, Spanish Civil War documents, notebooks, diaries, photos and more https://www.ucl.ac.uk/library/digital-collections/collections/orwell
Arthur Evans archive at the Digital Bodleian. Evans was an archeologist and museum curator who worked on Crete https://digital.bodleian.ox.ac.uk/collections/arthur-evans-archive/
Siegfried Sassoon (famous WWI poet and novelist, anti-war activist) journals and diaries https://cudl.lib.cam.ac.uk/collections/sassoon/1
Jeremy Bentham letters and manuscripts https://www.ucl.ac.uk/library/digital-collections/collections/bentham
The Carlyle Letters Online, between literary couple Thomas and Jane Welsh Carlyle https://carlyleletters.dukeupress.edu/home
The Correspondence of William Henry Fox Talbot, the inventor of photography http://foxtalbot.dmu.ac.uk/index.html
The Charles Darwin Correspondence Project https://www.darwinproject.ac.uk/ and Darwin Manuscripts Project https://www.amnh.org/research/darwin-manuscripts
Leigh Hunt Letters, 19th century Romantic poet https://www.lib.uiowa.edu/sc/leighhunt/
Letters from David Lloyd George to his brother, some from the late 19th century https://www.library.wales/discover/digital-gallery/archives/lloyd-george-letters
Roger Fenton’s letters from the Crimean War. Fenton was the first ever war photographer http://rogerfenton.dmu.ac.uk/
Roger Fenton Crimean War Photos at the Library of Congress: https://www.loc.gov/collections/fenton-crimean-war-photographs/about-this-collection/
Royal College of Nursing Digitized Collections, including some collections of personal papers of individual nurses, Florence Nightingale Records, magic lantern slides, and others. Browse the collection here https://rcn.access.preservica.com/archive/
Lewis Carroll Scrapbook at the Library of Congress: https://memory.loc.gov/intldl/carrollhtml/lchome.html
Charles Dickens manuscripts and letters at the New York Public Library https://digitalcollections.nypl.org/collections/charles-dickens-collection-of-papers-microfilm#/?tab=navigation and the Charles Dickens Letters Project https://dickensletters.com/
Virginia Woolf papers, including manuscripts, diaries, photos and more, New York Public Library https://digitalcollections.nypl.org/collections/virginia-woolf-collection-of-papers#/?tab=about
The William Blake digital archive, includes digitized manuscripts, prints, books, etc https://blakearchive.org/
The Collected Letters of Hannah More http://www.hannahmoreletters.co.uk/Letters/Book/Introduction.html
Thomas Hardy Correspondence Archive, https://hardycorrespondents.exeter.ac.uk/index.html
Webbs on the Web Project, the digitized diaries of Beatrice Webb (late 19th-early 20th c) https://digital.library.lse.ac.uk/collections/webb
The Gwillim Project, digitized letters, sketchbooks, and other primary sources of sisters Elizabeth Gwillim and Mary Symonds, who travelled to Madras, India in the early 19th century. These sources are useful both for social history and the history of science, natural history, botany, etc https://thegwillimproject.com/
Interested in Prime Ministers, Kings and Queens? John Major, Margaret Thatcher, and Winston Churchill papers are listed below in the politics and law category, as is a database of speeches by members of the British royal family