This is an interesting mark, Maxim. A number of British potters used their own versions of a Royal Arms mark, which could have been used with various patterns. I'm not finding any in our database with the particular features found on yours, the draped garter in the center and the chain attached to the unicorn's collar. Or the letters underneath! All we can do right now is keep the mark on file and let you know if a mark matching yours does show up. Thanks for sharing it!