What a breathtakingly enchanting place. I"ve long been familiar with the name due to Powys Castle Wormwood (Artemisia x 'Powys Castle'), but now I so want to visit. I might have spied the namesake wormwood in a distant border, but I definitely saw many other wonderful plants.
Lily-of-the-Incas (Alstroemeria cv.), Paperbark Maple (Acer griseum), Salvia x 'Amistad' (or similar) Chilean Bellfower (Lapageria rosea), Nerine bowdenii, Staghorn Sumac (Rhus typhina), perhaps a yellow Berkheya... Also Bartlettina sordida or a close relative.
There is indeed a yellow flowered Berkheya. Annoyingly they sell lots of plants from the garden in the shop, but not that one. I used to grow Berkheya purpurea(whatever), but it did not survive the move to here.