Bill,
Not disputing any of the historical information you provided. But in 50 trips to Europe, including the UK, since 2000, I have only been asked at an ATM if I wanted to pay a transaction fee by one in Barcelona. I declined and found another ATM where I was not charged anything by the ATM owner. So I have never paid any fee in Europe to get cash from an ATM.
You mentioned Monzo. I am not aware of a product like that in the US. What we have is various banks belonging to regional networks (regional is not very descriptive as most of the regional networks now have presence nation wide) where you are guaranteed no ATM fee for withdrawals. You can also get extra cash back when you make a purchase with your debit card at many grocery and other merchants for no fee. Otherwise, using an ATM other than one owned and operated by the card issuing bank will result in you getting charged a fee either by the ATM or your bank or both. Whether or not your card issuer refunds the fee is up to them. Most do not.
Curious about the £2.75 subsidy. Who pays that? Does this mean the rural ATMs will start charging that amount as a fee for withdrawals?
ATMs placed in fuel stations, grocery stores, bars, restaurants, and the like in the US that are not owned by that merchant pay a flat rent to the merchant. That plus they charge the maximum allowed by the card networks (currently $5) per transaction to the customer. Can make a considerable income from that.
US based banks, with the exception of Bank of America, have no agreements with any European banks to help avoid fees. And the BofA agreement isn't all that great because it only saves a small portion of the fees charged by BofA to their card holder. So, yes, asking someone at your bank here in the US which bank to use in Europe, or what the fee would be that the European bank will charge you at their ATM, if any, will be met be a blank stare. No US bank maintains that info. They should be able to tell you what they will charge you, their customer, for the foreign withdrawal, but that is about it.