If you are really worried your best bet is to move all your cash from Sterling into a foreign currency that you think will be resilient should Brexit occur.
I would avoid the Euro!
You could look at the US Dollar perhaps, make sure you are aware of the charges for moving the money over and back again, as you will at some stage probably want to get back into Sterling once it settles down, if it does indeed fall.
Based on my experience on the stock markets (I am not a currency trader) I would expect the pound to fall fairly sharply on a vote for Brexit and the Euro to do the same. Both would probably rebound quite quickly too as even if there is a Brexit vote it doesn't mean the UK Government will honour the outcome or take the steps quickly.
** I AM NOT A FINANCIAL ADVISOR AND HAVE NO QUALIFICATIONS AS SUCH **