Since then, there have been 16 more such events, or "Brexit indications."
A few examples: When Queen Elizabeth announced the EU referendum bill in a speech to Parliament shortly after the Conservative victory in May; when the U.K. independence party launched its Brexit campaign in September and when London mayor Boris Johnson announced his support for an EU-exit in February.
Around these events, markets have moved in some surprising and unsurprising ways.
Predictably, the British currency, the pound, comes under pressure as investors worry about the economic fallout of a Brexit. On days of Brexit indications, the pound loses ground against the the U.S. dollar more than 75 percent of the time. And one month later, the GPB/USD has been lower by 0.7 percent on average.
As of a few days ago, the pound had lost nearly 8 percent against the greenback since May.
But equity markets suggest something different — that many investors either aren't spooked by the prospect of Britain leaving the EU, or they simply don't think it's going to happen.
Unlike currencies, stocks haven't yet priced in much of a Brexit discount. On the day of Brexit indications, the S&P 500 has traded higher more than 75 percent of the time. Even the U.K.'s benchmark index, the FTSE 100, has traded positive 65 percent of the time.
British ADRs, however, have been rattled when Brexit fears rise. Big U.K. names listed on U.S. exchanges, such as Barclays and Aviva, have reliably underperformed the broader markets one week after Brexit indications.