A quick heads-up before diving into the story: I've set aside for you a complete shopping list of every ingredient you'll need so you can recreate this classic at home. Here's the tale.
The Bramble was created in 1984 by the Englishman Dick Bradsell, the same man who invented the Espresso Martini, at Fred's Club in Soho, London. Bradsell wanted to create "a quintessentially British cocktail," and drew inspiration from his childhood memories on the Isle of Wight, where he picked wild blackberries (in English, "brambles") in the late summer hedgerows.
The drink is a Gin Sour elevated with crème de mûre (French blackberry liqueur) drizzled in a "bleeding" pattern that creates the unmistakable dark purple streak in the glass. The Bramble was one of the creations that relaunched modern London bartending and transformed Bradsell into the most influential bartender of late 20th-century Britain. A Cool Britannia icon in the making, today an IBA official classic.
To recreate this one with the right style, tools matter as much as the recipe. I use and recommend a complete barman kit like this set on Amazon, which covers every essential tool at a very reasonable price. For a more serious setup, a professional cocktail bar station keeps bottles, glasses and tools organized and close at hand, instantly upgrading the whole experience.