I am shortly going on a Cruise with Princess Cruises who are the American arm of P&O, people who have been on cruises will know that on board is totally cashless so everything you purchase is put down to your stateroom and you pay the bill at the end of your cruise.
Being American the bill is normally in US Dollars but they have sent me an email saying that I can opt for the bill in Pounds sterling (at what they call a fair and acceptable exchange rate) normally I put the whole lot on a credit card and pay it off straight away and the card company make a conversion rate charge, so would be best to do Pounds or Dollars?
I have always found (in other situations) that it`s best to pay in local currency. I made the mistake of saying I would have my credit card bill in sterling in a duty free shop and the exchange rate was ridiculous. I prefer to let my credit card company do the exchange.
From my experience (with Princess) you'll get a much better conversion rate on your credit card than Princess offer so my advice is to get your bill in dollars.
My experience is that I always have it charged in dollars and put on to my Nationwide card. They convert it at a business rate which always seems good. I have cruised with Princess a lot but still feel I'm better with the dollar exchange through Nationwide.