Digital Collections of Primary Sources, War and Military, Colonial Violence in any region:
General Military History Collections -
Imperial War Museum Online Collections. Digitized archive of museum objects (like medals, photos, etc) but also some digitized archival material like letters, newspapers, and diaries. Be sure to click “Show only records with digitized media” when you search. https://www.iwm.org.uk/collections/search
National Army Museum London Online Collection. Digitized archive of museum objects and archival material – this is a better place to look for primary sources related to the 19th century empire than the IWM is. Be sure to use this link to see items that are available digitally. https://collection.nam.ac.uk/
Royal Museums Greenwich Online Collection, for material related to the history of the British navy, seafaring, “exploration” and more. After you search for a specific term, filter by “show only with images” to see items that are available online. https://www.rmg.co.uk/collections
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/
Collections related to particular conflicts -British Concentration Camps of the South African War (1899-1902) documents the world’s first organized wartime system of concentration camps, in which Britain held Boer (white South African) and Black civilians during a long and bloody conflict. These camps were known for their mismanagement. Thousands of people died of starvation and disease in them. Database developed by a team of South African academics. https://www2.lib.uct.ac.za/mss/bccd/
Intelligence Reports published by the Egyptian Army (the British Army in Egypt) during Britain’s long military campaign and series of wars in the 1890s fought against the Mahdi, an Islamist anti-colonial leader https://libguides.durham.ac.uk/asc-sudan-archive/sirs
*Not a collection of primary sources: Colonial Frontier Massacres in Australia 1788-1930, which identifies and visualizes sites of frontier massacres of Aboriginal, Torres Strait Islander and non-Indigenous people across Australia. This project uses primary sources to inform its analysis, but isn’t a database of primary sources. https://c21ch.newcastle.edu.au/colonialmassacres/introduction.php
Doing research on a particular British soldier -The London Gazette, an official government publication with roots in the 18th century. It includes War Office and Ministry of Defence notices, and is a record of military promotions, medals and mentions, etc. https://www.thegazette.co.uk/history
British Military Lists, digitized by the National Library of Scotland. “Military lists recording details of officers who served in the three main branches of Britain's armed services during the First and Second World Wars. Official lists for the British Army, Royal Navy and Royal Air Force have been published since the 18th, 19th and early 20th centuries respectively. Also includes unofficial 'Hart's Army Lists' of British Army and, from 1862, Indian Army Officers published between 1839 and 1915.” https://digital.nls.uk/british-military-lists/archive/97343435
See the National Archives Guides on how to do digital research related to the First World War https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/?research-category=first-world-war and the Second World War https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/?research-category=second-world-war-research