London dry gin emerged in 19th century London. It came about due to improvements in distillation, namely the invention of the Coffey still, which enabled a high quality alcohol to be distilled cheaply and efficiently. Gin had previously been sweet but this new spirit didn’t need sweetening to disguise the taste so the botanicals such as juniper could take centre stage.
Nowadays London dry gin doesn’t have to come from London, it’s a style rather than a place with an EU legal definition. It has to be made by distilling natural botanicals using neutral alcohol of 96% ABV. Nothing can be added after distillation except water. It has to have a minimum strength of 37.5% alcohol.