Quite possibly the first speakeasy to eschew the whole "serving you booze" thing, BYOC (Bring Your Own Cocktail) is a hidden bar that doesn't actually sell any alcohol -- instead, you bring the bottle of your choice with you, and they'll make it into a bespoke cocktail on the spot with their array of "syrups, fruit juices, spices, herbs, salts, and homemade cordials".
Head to this innocent-looking juice bar in Covent Garden, walk inside, and present your bottle (there's a Tesco 'round the corner where you can get some red rum, or whatever, if you forget) -- they'll let you behind the counter, and down the secret stairs at the back.
The space is a small but impeccably appointed candlelit throwback to the '30s era w/ a jazz-heavy gramophone.
After you've been seated, this trolley will be wheeled out to help get you, well, trolleyed, with the barkeep giving you a quick palate Q&A before getting to work on your drink.
£20 (per person) will get you two hours of boozy innovation, with a different drink each time, aided by bottles from other tables, from which everyone is expected to share and share alike... sure to make speaking quite easy indeed... and then much harder.