If you study the exchange rates you will find the pound started to decline in Nov 2015 from a high of 1.41 to around 1.26 by the time the referendum occurred. Market sentiment was that brexit would be a resounding nay(or neigh)yet the decline was ongoing. It was a market correction, as has been alluded to. To what extent brexit influenced the decline is impossible to determine.
Scroll down for pound euro and move the cursor on the graph for date and value.

https://www.exfinuk.com/historical-forex-gbp