Most ships work on a cashless system where your credit card details are taken and your then issued with a swipe card for each transaction you make on board. If you dont have a credit card then usually they will take pounds/dollars/euros (exchange rate can be very poor) at the start of your trip and you are then capped on how much can be spent.
Pounds sterling will be charged to your onboard account with Thomsons.
On a trip on Celebration in September the only thing I needed real money for was the casino and even there you can go to the cash desk for your chips and get them added to your account. I put my wallet, passport etc in the safe and forgot about them for the week. You will need some money for on shore cash purchases.
What ship and/or cruise line were you thinking of?