Cadog is a brand of Welsh butter. As the smart blue and gold packaging proclaims;
Menyn Cymreig o Hufenfa Llangadog - Cunhyrchwyd yng Nghymru
Welsh butter from Llangadog Creamery - Made in Wales
Like all proper Welsh butter it comes with the essential added ingredient of salt as the Welsh prefer their butter properly salted unlike their English cousins who generally take their butter plain. It is one of the few products sold in British supermarkets that features fully bilingual Welsh and English packaging. (The other one that springs to mind is Glengettie tea.)
Saint Cadog was a Welsh saint from the 6th century and after whom the town of Llangadog in Carmarthenshire was named and therefore an appropriate brand name for a creamery based in that town to adopt.
The Llangadog creamery is not an independent concern, it is part of ACC Milk a UK wide dairy business which makes a point of promoting the virtues of local brands. ACC Milk is itself part of the Co-operative Group, which perhaps unfortunately has decided to sell off its dairy manufacturing operation, leading to concerns that it will be brough by someone like Dairy Crest who will promptly shut half of it down.