You can take out up to £200 cash via an ATM fee-free (you get the exchange rate of the day so its usually better than getting your cash from a currency changer such as tesco - who charge commission).
Then all card payments, contactless or chip and pin are free of charge.
Hope this helps, it explains it a little more in detail in this post:
From today we’ll be introducing a 3% charge on cash withdrawals made from international ATMs, after a free 30-day allowance of £200.
- Spending by card in a shop, hotel, restaurant, or at any other “point of sale” abroad will remain free of charge
- Online transactions made in a foreign currency will also be free of charge
- We’ll continue to pass on the Mastercard® exchange rate with no markup
- ATM withdrawals in the UK will continue to be free of charge
I would say from experience, most shops and coffeeshops take card so its best to pay for everything via card (remember monzo will create a holiday summary for you - which is amazing). I would just take out enough cash for the munchies whilst walking the streets
Hope this helps