What is the greatest British Military victory.?

The Battle of Rorke's Drift. 22 January 1879, South Africa. The successful British defence of the mission station of Rorke's Drift, under the command of Lieutenants John Chard of the Royal Engineers and Gonville Bromhead.

Just over 150 British and colonial troops defended the station against attacks by 3,000 to 4,000 Zulu warriors.

Eleven Victoria Crosses were awarded to the defenders of Rorke's Drift,

Toss up between Trafalgar, Waterloo and El Alamein.

Running the yanks from Canada all the way down to the Gulf of Mexico and burning down the white house, with Madison scurrying naked through the swamp. WINNING the War 0f 1812.

The Spanish Armada was a huge threat in 1588 its defeat was a major victory.

The second Battle of El Alamein.  It successfully halted the Axis advance into Egypt and protected the Suez Canal.  

I'd say Trafalgar. Before it Britain had naval rivals. After it absolutely nobody could threaten British naval power until the early 20th century - and Britain couldn't have had an empire (or helped defeat Napoleon) without a dominant navy.

probably Waterloo

I would say the Battle of Britain too. A German military invasion by air and sea, 'Operation Sea Lion', would have followed quickly if the RAF, against the odds, hadn't defeated the Luftwaffe.

Battle of Britain

Sinking the Bismark.