I'm assuming the design is not carved into the wall caps...the surface is smooth?
This article describes how to create a moss slurry and paint it on to concrete to grow moss. The section about making designs is down a ways. I would think the UK would be a most hospitable climate for these creations. Very cool!
If this is how your example was created, looks like it took some effort and time!
Warning, this site is annoyingly ad-heavy
https://plantophiles.com/garde...
