The leaflet from the postoffice for counter sales is 2 April
But the link on line:-
Valid from 8 January 2024
ROYAL MAIL
ONLINE PRICES
Your handy guide to our online UK and International parcel and letter service prices.
I went through the Royal Mail site 9.14 am today and selected on line prices which clearly state 8 January
https://www.royalmail.com/sites/royalmail.com/files/2024-01/online-price-guide-january-2024-v1.pdf