See the purser and arrange to pay as you wish. do this mis cruise to avoid it appearing on your bill.
Both Thomson nd Island crusies are British and the tipping is more a US thing but now on most cruise ships but it never can be obligatory. I know NCL will add to your onboard account if not your final invoice but you can go to guest services and sort that out at the start of the cruise. The staff you will find in the restuarant and your cabin steward do a great job but you might be unlucky and feel the $10 is over generous.
If you read NCL terms & conditions, as of THIS YEAR, gratuities, now called a service charge, are NON removable. This charge can only be removed or adjusted in the event of a dispute. Costa cruises apply similar terms. As for 'it can never be obligatory' read reviews of people who have tried to get it removed on Costa, some have battled with their head office weeks after their cruise to no avail. Remember the cruise company have a hold on your credit card details whilst you are on board & can stick what they like on it. It would interesting to hear from people who have challenged the NCL rules in recent months since their new terms have been in effect.
I might be wrong, but I think Thomson include the tips in the price of the cruise - don't know about Island. Check the small print.