Unless there’s a delay or cancellation of Brexit, I’m fairly certain the main currency crash will be when the decision is made on the deal (or no deal) rather than on the day. If you went to Amsterdam right now and took cash out, you’d get the best rate on the first £200 worth of euros. There’s also a few euro dispensing cash machines in the UK that don’t charge fees, and those count as UK cash machines I believe and so won’t be subject to fees after £200 (if I recall correctly).
Currency exchanges tend to give poor rates, but it’s worth considering how much the rate will drop when the deal is announced and/or post-doomsday and whether that will compare to buying now from an exchange.
In short, it’s difficult to tell what will happen, which is why the exchange rates are so variable right now.