The currency on board is US$. So that is the only currency to take. Otherwise take your Nationwide card and pay with that.
Some say TCs are old fashioned but I find them a secure way to take money and if you take US Dollar TCs there is no fee or commission to change them on Celebrity ships. By all means take some dollars as you can spend them just about everywhere and do take your Nationwide card. When you come to pay your final account on the ship ensure (preferably ensure earlier) that you want to pay in US Dollars and let Nationwide do the conversion for although Celebrity's rate is reasonable Nationwide's is better. You don't need to take huge amounts of TCs but with the exchange rate being as it is it's money in the bank, if you can afford it, for future times. Neil Dow.
I've just looked and I think you are going to the Med with Millennium so I would certainly take some Euros and top up from an ATM with Nationwide as and when. US $ are not important on this trip unless you intend to pay your tips individually and even then the UK£ or the € equivalent is ok.. ...Neil Down
ps The ship name has 2 Ls and 2 Ns ....cruise.co.uk have it wrong, it's Millenniu.