Don't mention it
Show it
Suspension of disbelief is a powerful drug. It clouds the mind and prevent the reader from realizing that Quimbonia is not a real country. It's true that it looks and feels like the United Kingdom. Its inhabitants even speak British English, and each carry an umbrella. Fact is, it is called Quimbonia and has no coastline.
To achieve this result, you need to be the first to suspend your disbelief. Convince yourself that your world is real, and write your story in it with the same tone and ease as if you were writing about our Earth. If you are the first to find it hard to believe in your world, there is no reader that will fall for it.
The other important element is that you need to give the reader the cues of what they are seeing and feeling. Even the real world would feel fake if you don't show it to the reader. For instance, saying "they were in Paris" is not the same as describing the current season in Paris, how heavy their clothes are, how the wool scarf in pinching on the neck, and whether it is a murky day, with smog low over the city, and the Eiffel Tower piercing right through the grey clouds. Even a well known landmark like the Eiffel Tower could be left as a name, or showed to the reader as a slender elephant of steel, built for the vanity of mankind.
What if my world is like Earth, but different?
That's the easier thing to do. You won't need to give names to seasons, or show that the sky was neon green. You can directly use common things from our world to make your fantasy become believable. For instance:
Keith Coleman held his first prototype for a light bulb in his hand. It had been ten years now since Quimbleton had been the first town to fend off the darkness that creeps at night. The whole world envied them both, they envied the inventor for his skills, and they envied every single red brick of the small, industrial, greasy town for not deserving one such great man.
The difference from your text is that I have tried to show you the man and the town, and place them in relation to their world. Now you have a mental image of it. On the contrary, recording some fact, which you may expect the reader to care about and perhaps even remember, is less likely to leave a trace. If you need to give a fact, do so in a dialogue
"Keith Coleman from Quimbleton! The world owes you, and not just for the invention of the light-bulb." cheered UnimportantCharacter