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The Nationwide CC gives you the commercial exchange rate rather than the tourist rate, usually around 5 cents more. Why take TC's when you can use a CC although I am sure you will be able to change them.
To add to the answer, quite apart from using a more favourable exchange rate, Nationwide does not charge any commission when charging a foreign currency transaction to its credit card (and I believe the Saga card is the same). But you will not get these advantages if Celebrity charges your card in your own currency (£) which it says is the normal arrangement. It cannot insist on this and you can opt out (at check-in time) and will almost certainly get a better deal if you ask to be charged in $US. As far as I can tell, there should be no requirement at all for any dollars on board the ship and the only currency required will be that for on-shore visits - can anyone confirm this? Local currency is probably available on board but at a poorer rate than using a debit card on shore, and with a charge for using the cash machine. Here again, the Nationwide debit card is commission-free. (No, I don't work for Nationwide!