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?
A planet with a heavy greenhouse effect can have it.
Think about a planet which is at the far ranges of goldilocks zone, like Mars. It compensates the low insolation with high greenhouse effect, like Venus. As a result, planet's climate is much more uniform than Earth's. Difference between equatorial and polar regions is minimal. However, this planet should have a heavy cloud cover, and sunshine would be rarely seen on the surface.
In general, you can't have the same climate on whole planet.
- There are always areas at equator which would got more sunlight and therefore have different climate.
- Britain enjoys warm Gulf stream which makes climate more warm. Compare climate at London at 51°30′ N and other side of Atlantic ocean: coastal Saint-Augustin at 51°25′N with subarctic(!) climate. The Quebec at 46°48′ N is much closer to the equator and also has colder (Dfb class) climate than London.
- Far away from sea climate is more arid and summer is warmer. In other words, you have continental climate away from sea :)
If you really want to do so, you could describe really strange world
- Without continents but with big islands.
- With mountain area near equator (the area would got more sunlight but when altitude grows, climate become colder) you could spread desired climate to one direction.
- With warm oceanic streams near poles you could spread desired climate to another direction.
But I suppose that such world is so artificial that it's hard to believe in it.
On the other hand you have another options:
- If your setting is close to our middle ages then people do believe that (relatively small in terms of planet) area 1000x1000 km (which is big enough for many countries) is
entire worldand nobody cares what goes on somewhere else.
- If your setting about high-tech then terraforming was performed (either by people or by an Ancient civilisation) and nobody cares about details.
- If your setting allows magic then ... it's a magic! Very strong and arcane magic, of course.