In my infinite wisdom, we booked an Amsterdam holiday for April 1-6
Obviously, apocalypse day is two days before we leave
It’s probably wise to take out some spending euros nowish rather than spend it all on my card as I’d planned to, right?
Am I better off going with an exchange or can I take out Euros here with my Monzo card somehow, is that rate better?
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.
I’d get 50% of what you think you might need now. That way you mitigate any possible losses whichever way the exchange rate goes.
I would get a Revolut account and use it to exchange currency at the interbank rate (during weekdays).
If anyone on here could give you a definitive answer they would be rich and not here in the first place. If you do manage to out-guess the forex market then you will be wealthy beyond your wildest dreams 
It’s a question on timing the market, which even the best can’t do. Essentially it boils down to your opinion.
My opinion, and that’s all it is, would be the following based on these scenarios and my perceived outcomes:
Brexit gets cancelled: Pound strengthens
People Vote announced: Pound somewhat strengthens
Deal reached: Pound Somewhat Strengthens
Article 50 extended: Pound stays the same.
No Deal Brexit: Pound weakens.
Depending on how you feel on each of these points and which you feel would be the most likely should help you decide when to buy your Euros.
Logically, but regrettably not wrapped in political nuance, this is the answer. 
Probably true.
To get absolute average, take the weeks between now and when you head off and divide by the amount you’re planning to exchange. Then buy that amount per week.
Essentially, dollar cost averaging - a John Bogle (RIP) concept. (I think).
You should stock up on tinned food, too.
