It used to possible to foreward mail to someone by crossing out the address, writing the forwarding address, then popping it back in the post without a stamp. However, I recently learnt that the regulations changed in 2018 to say the Royal Mail are under no obligation to do this.
"No obligation" is obviously different to not allowing it at all - does anyone know what happens in practice? If you've recently forwarded mail and know it got to its destination, please let me know.