Events from the year 1812 in the United Kingdom. The United Kingdom is still involved in the Napoleonic Wars with France and its attempts to stop French trade lead to the War of 1812 with the United States. Arthur Wellesley, Marquess of Wellington, is active in the Peninsular War in Spain. The year also marks the only assassination of a Prime Minister of the United Kingdom when Spencer Perceval is shot.
- February - Framebreaking Act imposes the death penalty for Luddites.
- 27 February - Poet Lord Byron gives his first address as a member of the House of Lords, in defence of Luddite violence against Industrialism in his home county of Nottinghamshire.
- 15 March - Luddites attack wool processing factory of Frank Vickerman in West Yorkshire.
- 16 March–6 April – Siege of Badajoz (Peninsular War): The Anglo-Portuguese Army, under the Earl of Wellington, besieges Badajoz in Spain and forces surrender of the French garrison.
- 4 April - U.S. President James Madison enacted a ninety-day embargo on trade with the United Kingdom.
- 11 May - Bankrupt banker John Bellingham assassinates the Prime Minister Spencer Perceval in the lobby of the House of Commons.
- 25 May - Felling mine disaster: Mine explosion at Felling colliery near Jarrow — 96 dead.
- 8 June - Robert Jenkinson, 2nd Earl of Liverpool (Tory) becomes Prime Minister. Aged 42 at accession, he will hold the office for almost fifteen years, the second longest-serving Prime Minister of the United Kingdom after Pitt the Younger, who was in office for a total of 30 years across his two premierships.
- 18 June - The War of 1812 begins between the United States and the United Kingdom.
- 18 July - The Treaty of Orebro brings an end to the Anglo-Russian War and the Anglo-Swedish War.
- 22 July - Peninsular War: At the Battle of Salamanca, British forces led by Lord Wellington defeat French troops near Salamanca in Spain.
- August - Henry Bell's PS Comet begins a passenger service on the River Clyde between Glasgow and Greenock, the first commercially successful steamboat service in Europe.
- 12 August - Peninsular War: Wellington enters Madrid following the Battle of Salamanca.
- 16 August - War of 1812: American General William Hull surrenders Fort Detroit without a fight to the British Army.
- 19 August - War of 1812: USS Constitution defeats the British frigate Guerrière off the coast of Nova Scotia.
- 5 October–10 November: A general election sees victory for the Tory Party under Robert Jenkinson, 2nd Earl of Liverpool.
- 9 October - War of 1812: In a naval engagement on Lake Erie, American forces capture two British ships, HMS Detroit and HMS Caledonia.
- 13 October - War of 1812: Battle of Queenston Heights - As part of the Niagara campaign in Ontario, Canada, American forces under General Stephen Van Rensselaer are repulsed from invading Canada by British and native troops led by Sir Isaac Brock (although he dies during the battle).
- 7 November - HMS Victory is retired from front-line service.