I haven't sailed with Thomson but I would have thought that they would bill accounts in £Stg. If this is the case then the advantage of a Nationwide account is not an issue as it only comes into play with regard to charges when using it to obtain currency from at ATM abroad or purchasing items in shops abroad. In these instances Nationwide don't impose a fixed charge or percentage charge for foreign currency transactions. I'm assuming you have a FlexAccount debit card and wish to pay your on board account using that card. As ships aren't connected directly into the banking system then PIN numbers are not an issue (they don't work). They will simply swipe your card and do the billing transaction when the ship ends its voyage. This implies that, for a debit card, they are taking a chance that you have the money in your account and for this reason some cruise lines (eg Costa) don't accept debit cards at all. So, as Neil Down has suggested, read the small print. Have a nice cruise.
I would suggest taht you read the small print in the "condition of booking" as it will tell you there. ... Neil.