Plausible reasons for a whole planet to have a temperate oceanic climate?

In my book series, I'm worldbuilding a planet whose culture is based on Celtic mythology and history, and in order to be consistent with real life I feel like the whole planet's environment needs to mirror that in which the celts (particularly the insular ones) lived in irl: forests and grasslands with a temperate oceanic (Cfb) climate dominating the vast majority of the landscape like it does in Britain. I do allow small exceptions for mountain ranges, peaks, and slightly hotter and drier areas (which can be given subpolar oceanic, tundra, and warm-summer mediterranean climates respectively) but what circumstances could plausibly cause an earth-sized planet to be this temperate and wet all around?

posted over 4 years ago

The Weasel Sagas‭

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