Not sure if its correct but I was once told by an American its against US law to charge credit card fees, they couldn't believe we were charged fees for using cc's, I know I have never been charges cc fees for paying anything in $US's, including Disney cruises.
Any fees will be by your credit card, not by DCL. Which card do you have, I only use Post Office or Halifax Clarity for US transactions, no fees at all, others you will be charged upto 3%.
No idea about DCL in £'s(I always book cruises in $'s, as I have for next year)
Their T&C's used to be better when booking in $'s compared to £'s but have to admit I didn't check this time, I just automatically book in $'s now.