This is not a rule or an agreement between NR and the trade unions, NR is simply following the employment laws on the statute books, where by an employer has to give the employees days off for bank public holidays unless they can’t then they must give a competently Fay off in lieu instead. If there is no work for you then the company is quite entitled to book you off and pay you a days wages. If I could be bothered I would look things up in my employment law books from LRD which are a great source of information when it come to employment law.
https://www.lrd.org.uk/ One of the first things that was taught at my unions reps level one course was that any company has the right the manage it’s business and finances anyway it sees fit as long as it’s within the law regardless what trade unions think. The company always has the upper hand.