I actually have been able to get out of Dynamic Currency Conversion when paying with my American Visa card, but it's tricky. At the bottom of the payment page, after you put in your address, you'll notice a line that says Total Amount, then "Including Administration Fee" in red, followed by a space, followed by the amount in the home currency.
After you put in your credit card number and tab to the next box, the amount they intend to charge you in your currency appears between "Including Administration Fee" and the total in the home currency. If you click the link that says More Information, next to that amount, you're taken to a screen where you can untick a box to just be charged in the home currency, and let your bank exchange it at the right rate.
The caveat is that I've only tried this with an American credit card. I don't know if this holds for a Euro card paying in pounds.