Data paint for UK traveller to
Mexico with a Santander UK current account and/or a Santander UK Forex free Credit Card (but this could also be valid for other worldwide Santander customers, worth checking within your country’s Santander T&C):

if you have (or worth opening) a Santander account that allow free cash withdrawals at any Santander’s worldwide ATM (e.g the legacy 1-2-3 and Select 1-2-3 accounts, the recent Private and Edge accounts, and or one of the Forex Free Credit Card like the legacy free fee Zero or others with a monthly fee), you can use the Santander ATM’s in Mexico for free, but make sure you always reject the proposed conversion on screen.

PS: I have also tested this in Portugal, Spain and some years ago in NYC and Miami, the main reason why I keep my legacy account active.