What is the fee for transaction? In a store for example outside UK?
Mastercard exchange rate but NO fees on top
What is the ATM fee outside UK?
It is currently FREE but it costs Monzo about 1% if in Europe and 2% elsewhere and it looks like they will be introducing a 3% fee shortly
Everyone can use Monzo?
USA people also?
If you mean resident in USA then no, but if you mean US citizenship then if they resident in UK then they can have an account
What is the fee for currency exchange?
Mastercard exchange rate
Should I use only GBP as main currency or USD or does not matter?
Always use the currency of the country you are purchasing in and decline Dynamic Currency Conversion