I'd reject The Magna Carta - 1215 as in reality too limited a reform which at the time affected only the aristocratic class. I'd also reject Women's Suffrage -- 1928 as nothing to crow about since other countries had granted women the vote years earlier. I'd also reject The NHS -- 1948 on the grounds that other European nations were setting up socialised health care at the same time. I wouldn't be tempted by The Abolition of Slavery -- 1833 either, on the grounds that it is equivalent to a husband being proud of having stopped beating his wife.
Of the military years, Agincourt -- 1415 is out -- there is nothing about the 100 Years War to feel proud about. The defeat of the Armada -- 1588 had more to do with luck than judgement. I've changed my mind about the Falklands War; at the time I was anti, but since then I've come to think Thatcher was right. Nevertheless I would not pick 1982 as our finest hour. I reluctantly reject Waterloo, since our victory was a jointly shared with our allies, although the lion's share of the honours go to Britain and the victory ensured that the nineteenth century was Britain's century.
That leaves one "social" year and one military: The Bill of Rights -- 1689, and 1940, but I can't decide between them.