Just got back from the UK and Ireland where we spent 2 weeks on vacation. We had notified our bank and Credit Card companies that we would be overseas for these weeks, so they would not jump up right away and block the cards when transactions started coming in. I got 50 Pounds from an ATM at Heathrow when we landed just to have some cash. No ATM fee there , but the exchange rate was $1.77 per Pound. Ouch.
I found that many of the banks offered no ATM fees and the best rate was at the Tube stops, at $1.68 per Pound. My bank charged $1.50 for each withdrawal, so I made sure to get 200 or 300 Pounds at a time to keep the fee cost as a percentage of the withdrawal low.
We used cash most of the time, and when we had to use a card, we used the credit card with the chip. If there were an suspicious charges, they can be disputed. Not so much on a debit card. All of the restaurants bring the credit card machine to the table so you can see the transaction being handled.
We made sure to use up most of our cash before we left the UK for Ireland and their Euro.
If you do go from the UK to Ireland and plan to take some bus tours. Some of the tours go to Northern Ireland (The Giants Causeway for one) and you will need Pounds there.