Britain's FTSE 100 fell 26.45% on Black Monday, October 19, 1987. The decline was the largest one-day percentage fall in the index's history and exceeded the same day's percentage drop in the U.S. Dow Jones Industrial Average.
The crash was international rather than confined to Wall Street. Hong Kong shares had already plunged, European markets followed, and U.S. trading later produced the Dow's 22.6% fall. Investors faced fears about valuations, interest rates, currency tensions, and the ability of computer-assisted trading strategies to amplify selling.
The FTSE 100 number is an index percentage change, not a fall in the value of every British company by exactly that amount. The index measures the performance of major companies listed in London, and its composition has changed over time. That distinction explains why historical index comparisons require attention to the measure being quoted.