It was popularised by cooks from India living in Great Britain, but I don’t believe that makes it any less Indian. Just as the chimichanga wasn’t invented in Mexico, but is still considered Mexican food.

If by common knowledge, you mean that a significant portion of the population believes it, I’m not sure how reliable that evidence that is. People will believe a whole lot of strange stuff.

On topic, even the first paragraph of the Wikipedia page states that it was “popularized by cooks from India living in Great Britain”. Regardless of where it was first created, this is clearly the product of Indian immigrants. I don’t believe their heritage should be ignored just because they moved. Although, I don’t want it to sound like I believe in a 100% black and white distinction here. It’s clearly a fusion dish with British influences. The original chicken tikka was a lot dryer and the “masala” sauce was added to make the dish creamier to appeal to British tastes.

However, I don’t go around claiming General Tso’s chicken isn’t Chinese food, just because it was first made in New York; or that the chimichanga isn’t Mexican food, just because it was originally made in Arizona; or that a Cuban sandwich isn’t Cuban, just because it was first made in Florida. These dishes wouldn’t exist without the immigrants who modified their cultural recipes to adapt to a new environment.

To me, chicken tikka malala is an Indian dish with British influences.

E: Tao to Tso.