Posting kitchen knives using Royal Mail is perfectly acceptable and the below is taken directly from their own website:

“Sharp objects like knives, kitchen utensils and gardening tools may only be posted if they are packaged appropriately so that they are no risk to employees, other postal items or recipients.

Wrap heavy cardboard around sharp edges and points, strong enough to ensure that the contents do not pierce the outer packaging. Wrap each item with cushioning material. Place in a suitable outer container such as a padded envelope.

The sender’s name and return address must be clearly visible on the outer packaging.”

You can view this information on the Royal Mail website by visiting the following link and looking under the section titles ‘Sharp Objects’ near the bottom of the page:

https://www.postoffice.co.uk/mail/what-can-i-send