NCL does a lot of marketing to the UK for its voyages. Do they offer UK passengers a different payment arragement that includes the $10 daily service charge as part of the overall cruise price?
The "hold" on your account refers to paying with a debit card -- not a credit card. Your credit card account would not be put on hold. Different story if using a debit card since the money comes right out of your account immediately and there needs to be enough money there to cover whatever is charged to the account. To make sure that the money is there, a "hold" is placed on your account for an amount that would cover the service charge or one of more passengers in a cabin that are using the same card for that cabin.
Hope that helps!