IIRC, England (Anglican Church) at one time designated 2 days a week as fish days to help out the fishing industry. It wasn’t penance. Maybe 200-300 years ago. I no longer have the book but remember it because I’m Catholic and it caught my eye.
It’s far from unnatural that something like this could and does happen when the head of the government and the head of the church are the same person. And, as I’m sure you’re aware, the Church’s prohibition is against meat, not pro-fish. The idea was to abstain from what constituted the high point in the daily diet. Once lobster thermidor, clams cassino, trout meuniere, etc., became available via refrigeration/electricity, abstaining from “meat” became less a sacrifice. Even so, it was as vivid a sign of Catholicism as ashes on the first Wednesday of Lent.
You may have heard the story, but I love the one about Perry Como who complained that the dinners to which he was invited invariably fell on Fridays. His stock line was, “I’ll have a poached egg on toast, and please serve it HOT.” Even if it wasn’t always a great sacrifice, it was still an undeniable profession of faith.