I would say be wary, but specify upfront that you want to be charged in pound sterling. My most recent trip to the UK was three years ago, and the only place I got hit with DCC was at the Heathrow Marriott. Hotels, department stores, and airport duty free shops are notorious for DCC, so I'd be especially vigilant both when checking in to a hotel and when making purchases. I've read reports that Marriott is bad about DCC in the UK, but I can't speak for the other hotel chains. I made plenty of purchases in the UK on my credit card, and I didn't remember DCC being too prevalent there.

I can't speak for Ireland as I've never been there, but I do know it was the birthplace of DCC. Even fast food restaurants like Burger King pull the scam there. The worst part is that for transactions like that the dollar value is below the threshold requiring cardholder verification, so you just get handed the receipt and it's a done deal. Fortunately in those cases US card issuers have been quick to issue courtesy credits. While the cardholder doesn't lose out in this case, I still am unsatisfied with the resolution because the merchant doesn't get hit with a chargeback.

Aside from transactions where there won't be cardholder verification - like American fast food joints in Ireland - I would say that you'll have no problems opting out of DCC in most places as long as you specify pound sterling upfront in the UK or euros in Ireland. In fact, some waiters and cashiers will give you a confused look that says, "What else would I charge you in?" but it's better to specify upfront than to find out the cashier can't void the transaction after the fact. (Fellow member percysmith found this out at Harrods.)

If faced with a forced DCC situation, kindly ask the cashier to void the first transaction and walk you through the steps. If the cashier refuses to void the transaction, deface the receipt by circling the local currency amount, writing "merchant refused local option/DCC forced", and crossing out the exchange rate. I highly doubt it will come to this last case, but if it does please come back and we can help out.

I would say restaurants aren't likely to be a problem because at many places the waiter brings the payment terminal to the customer after the meal. You get to handle the terminal during the transaction, and any compliant terminal would offer an opt-out from DCC - assuming it's compliant that is. If you're presented with a question like "DCC Accept?" or "USD/EUR 1.333" or "USD/GBP 1.539 OK?" or anything that looks like DCC or an exchange rate, press the red cancel button immediately. It's a counter-intuitive process, and this is part of the reason why DCC is so sneaky.

Hotels are much less likely to be a problem. Just make sure you don't use express or quick checkout. Always go down to the desk and make sure they run the card on file and you get the final receipt from the POS terminal or their payment system that shows billing in local currency. If there is any mention of an exchange rate on the folio, demand that they void that transaction and rerun it. Check everything twice before you sign. (But also check to make sure the room rate is correct. Don't be like me in Hong Kong last week where I was so fixated on making sure I didn't get DCC that I didn't notice they had overcharged me by mistake! )

Please report back on your experiences and enjoy your trip!