It does indicate DCC preference, and Marriott properties in the UK all have DCC. (I've been hit before.) It's been awhile since I've seen the prompt, however. As a best practice, I preauth the room with an AmEx and then settle the bill with the card of choice. For Marriott properties, this is the SPG AmEx, so no card change is currently required. Before the merger, I would switch to the Chase Marriott card upon check out.