I suspect Royal Mail itself is experiencing staffing issues. I placed an order with them in September. Normally, they send an email informing of the order has been processed and despatched. I did not receive that. I sent an inquiry. I received confirmation of it, but no reply. I sent a second inquiry and within two hours, I received confirmation the replacement order - the original had never been processed - had been processed. One or two days later, I received notice of despatch and within five days, I received the stamps. During this period, the Duran Duran prestige stamp book was issued and received. So, there are no problems with mail from the UK to the Netherlands. I, also received an auction lot within a week of bidding, during this period.
Also, you can see it on their shop website.

Unlike the other items, the prestige stamp book is not available yet.
Some dealers receive items ahead of the issue date. Maybe, the issues occurred during printing and dealers received books from the initial printing or loose panes to service their first day covers.