Key Digital Collections of Primary Sources - General Britain and British Empire:
A Vision of Britain Through Time. Maintained by the University of Portsmouth, this site offers historical maps, statistics culled from census records and election results, and an archive of historical travel writing. https://www.visionofbritain.org.uk/
Victorian Web. This website can be tricky to navigate but has some really great primary sources and introductory secondary articles on various subjects. Start here on the “how to use this site” page: https://victorianweb.org/misc/vwintro.html
British Library Digital Manuscripts. Many of these are from before our time period, but it’s a fantastic resource. https://www.bl.uk/manuscripts/Default.aspx
Internet Sourcebooks. This is a collection of pretty well-known primary sources (the kind you might have to read in a gen-ed course), but can be very useful if you’re looking for a major speech by a politician, a law or legal document, etc. https://sourcebooks.fordham.edu/mod/modsbook20.asp
Europeana. This is a massive collection of digitized primary sources funded by the EU and has a little bit of everything, from letters and diaries to images and museum objects. There is a lot here – European museums and archives upload and share their materials on the database. Start with the How to Search page https://www.europeana.eu/en/help/search-tips or visit the 19th century collection https://www.europeana.eu/en/collections/time/19-19th-century. You can also sign up and make your own “collections” of documents.
British History Online 19th Century Primary Sources, via the Institute of Historical Research. Go here for London history, parish registers, etc. Lots of pre-19th century material. https://www.british-history.ac.uk/catalogue/19th-century
Chief George Manuel Memorial Indigenous Library, digital collections related to indigenous people around the world https://www.cwis.org/chief-george-manuel-memorial-indigenous-library/search/
Disability History Museum online collections https://www.disabilitymuseum.org/dhm/lib/overview.html
Digital Bodleian (Oxford University Library) collections https://digital.bodleian.ox.ac.uk/ Be sure to check out the Modern Digitized Manuscripts, which include many sources related to the British empire, including the papers of the famous archeologist Arthur Evans, the personal archives of various British Colonial Administrators (especially those who worked in Africa), and more https://digital.bodleian.ox.ac.uk/collections/modern-manuscripts-and-archives/
UK National Archives, only some items are available online in digital format, this link will show you which. There are lots of records available for the Army and Navy. Some very relevant collections include the Victorian Prisoners Photo Albums, Famous Wills, and Logs and Journals of Ships of Exploration. https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/?letter=&search=&research-category=online
National Portrait Gallery, London. For painted and, later, photographic portraits of prominent Britons and colonial subjects. https://www.npg.org.uk/collections/
Royal Commonwealth Society Digital Archive, via Cambridge University. Includes 19th century image collections from South Africa, British India, Australia, New Zealand, New Guinea, and the British Caribbean colonies https://cudl.lib.cam.ac.uk/collections/rcs/1
18th Century Entertainment Ephemera Collection at the Digital Bodleian. Includes advertisements and programs for animal exhibitions, concerts, circuses, dioramas, etc etc mostly in Britain https://digital.bodleian.ox.ac.uk/collections/entertainment-ephemera/
New York Public Library Digital Collections; search for terms like Britain, British, English, etc for relevant items. Includes a lot of natural history texts https://digitalcollections.nypl.org/
British Library Victorian Britain Collection Items, available digitally online. If you’re feeling lost, this is a good curated collection to start with https://www.bl.uk/victorian-britain/collection-items
Other helpful resources for British history projects:
HISTPOP, database of historical population data in Britain http://www.histpop.org/ohpr/servlet/
Historical Currency Converter, via The National Archives London https://www.nationalarchives.gov.uk/currency-converter/