What happened in this bunker had never happened before in human conflict. Air Chief Marshal Sir Hugh Dowding had spent the late 1930s assembling the world's first integrated air defence system. He connected Chain Home radar, the Observer Corps, anti-aircraft batteries, barrage balloons, intelligence services, and Fighter Command into a single information network feeding into rooms exactly like this one. Plotters - many of them Women's Auxiliary Air Force - received reports by telephone and moved numbered wooden blocks across a map table to show enemy and friendly formations in real time. A tote board displayed each squadron's status with coloured lights: at standby, enemy sighted, ordered to land. Coloured discs indicated weather at the sector airfields. A sector clock with coloured wedges tracked time. The controller standing on a raised gallery could absorb the whole picture in seconds and decide which squadrons to scramble, in what numbers, from which airfields, to intercept which raid. Before this system, fighters scrambled blind. After this system, they could be vectored onto incoming bombers with seconds of warning.

Excavations began in 1938, shortly after the Munich Crisis convinced everyone that war was coming. Sir Robert McAlpine - a civilian contractor, because the project had to look ordinary - built the bunker between February and August 1939. The Operations Room sits sixty feet below ground at the bottom of a staircase of seventy-six steps, with thirty feet of earth above the ceiling. The walls, floor, and ceiling are about a metre thick, concrete with waterproof lining. No bomb of the period could penetrate it. Every utility - electricity, water, telephone, sewage - runs down those seventy-six steps. The ventilation and air filtration system installed in 1939 still works today. The whole installation became the prototype for the other five Group headquarters scattered across Britain: No. 9 at Barton Hall, No. 10 at Box, No. 12 at Watnall, No. 13 at Blakelaw, No. 14 at Raigmore. But Uxbridge ran No. 11 Group, which covered London and southeast England, and No. 11 Group saw most of the fighting.

The bunker's first operational test was a disaster. On 6 September 1939, three days into the war, Chain Home radar reported incoming enemy aircraft over the Thames Estuary. The No. 11 Group controller, working underground at Uxbridge, scrambled 56 Squadron and 74 Squadron to intercept. The radar plot was a misread of friendly aircraft. Spitfires of 74 Squadron then mistook the Hurricanes of 56 Squadron for the enemy and shot two of them down. Two RAF pilots died - the first British air casualties of the Second World War, killed by their own side, in what became known as the Battle of Barking Creek. Their names matter: Pilot Officer Montague Hulton-Harrop was the first British pilot killed in the air war. The system that would later win the Battle of Britain killed its own people in its first engagement. Lessons were absorbed quickly. Identification procedures tightened. Radar operators learned to recognise friendly returns. Air Vice-Marshal Keith Park arrived in April 1940 as Air Officer Commanding No. 11 Group and the bunker began running tactical operations that worked.

Of 1,733 German aircraft lost during the Battle of Britain, No. 11 Group accounted for the majority. The Luftwaffe attacked British ports first, then on 12 August 1940 attempted to destroy the Chain Home radar masts - and largely failed, because the wooden mast structures simply absorbed bomb damage. The following day, Eagle Day, they switched to attacking the airfields themselves. The King and Queen visited the bunker on 6 September 1940. After the Luftwaffe shifted target to London on 7 September - the start of the Blitz - the pressure on Park's pilots eased fractionally, though London paid the price. Churchill's second visit on 15 September is the famous one, the day every bulb glowed red. The pilots who flew that day, mostly in their early twenties, were the men Churchill meant when he said "never in the field of human conflict was so much owed by so many to so few." Many of them never reached thirty. In December 1940 Air Vice Marshal Sir Trafford Leigh-Mallory took over the Group and modernised the plotting system. Operation Jubilee at Dieppe in 1942 was controlled from this room. So was the air cover for D-Day in June 1944.

RAF Uxbridge today sits about 14 nautical miles west of central London, on the edge of the urban sprawl that has long since swallowed the airfield itself. The bunker entrance is marked by a Spitfire gate guardian - a replica Mark IX painted in No. 11 Group colours. Hillingdon Council runs the site as a museum, with the original Operations Room restored to its September 1940 configuration: the map table, the tote board, the sector clock, the coloured chairs of the plotters. A visitor centre above ground opened in 2018. The bunker lies under Hillingdon Borough at 51.5412 N, 0.4653 W. RAF Northolt - still an active military airfield - lies three nautical miles east. Heathrow is six nautical miles south. The airspace that No. 11 Group once defended is now the busiest in Europe, with passenger jets stacking over the same patch of sky where Park's Hurricanes once climbed to meet German bombers.