If we took a look at the Scottish Independence Referendum in 2014, we can see how similar are these two events.
About 1 week before the voting, the "remain" league takes the lead, and GBP gains by a huge percentage.
So, the voting day is not important, the day when the result is available is IMPORTANT. We know that there are 2 results: 1. UK remains to be a member of EU. This is the same as Scotland remains a part of UK. But the gains over the 10-day period was extremely good (too good actually), so people began to take profit, and the drop began. 2. UK leaves EU. No doubt, short GBP.
In addition, the gap is still way below the current price. So, it seems that either way, we should find good entrance to SHORT GBPJPY. :)