The story
SW1A begins the postcode of many government addresses, most famously the House of Commons (SW1A 0AA), Buckingham Palace (SW1A 1AA) and Downing Street (SW1A 2AA). See SW1.
Etymology from Wiktionary, CC BY-SA 4.0.
SW1A
Today it means: Synonym of SW1; British government institutions as a whole. Full definition →