text: https://
  www.royalmail.co
  m/sites/
  royalmail.com/
  files/2022-04/
  royal-mail-prohi
  bited-and-restr
  icted-items-apr
  il-2022.pdf
  The information is on the Royal Mail website. You can send over 1 litre with Royal Mail within the UK (not overseas) providing it is secured packaged in a plastic sealed bag. Details in the above PDF link, first page bottom right.