Some background history here:
http://www.gbps.org.uk/information/rates/inland/printed-matter.php
It's also worth remembering that the general structure of international postage rates (e.g. what can be posted at a discounted rate, such as printed papers) isn't determined by Royal Mail or Parcelforce Worldwide but by the Universal Postal Union, which is a specialised agency of the United Nations:
https://en.wikipedia.org/wiki/Universal_Postal_Union
(The Post Office has absolutely nothing to do with postal rates, either within the UK or for international mail. It's no longer part of Royal Mail).
To find the best rates for sending parcels you should always go through a parcel broker. Even if you end up taking your parcel to a post office (to use the services of Royal Mail or Parcelforce Worldwide) you can often save a significant amount of money by pre-paying through a brokerage.
https://www.parcel2go.com
https://parcelbroker.co.uk
https://www.parcelhero.com
https://parcelcompare.com