Back in 2008, I bet:

If any current EU member with a population over 10 million people
in 2007 officially withdraws from the EU before January 1, 2020, I will
pay you $100. Otherwise, you owe me $100.

Before the June 23 Brexit referedum happens, I want to clarify that a majority vote for “Leave” is not sufficient for official withdrawal.  I will happily concede defeat if and when (a) the European Union officially removes Britain from its list of members, or (b) the British government officially announces a unilateral withdrawal.

If any of my betting partners objects, I am willing to defer to a mutually acceptable arbiter.