I was about to say this same fact on another comment above, but then I read the wikipedia article again

apparently it isn’t like an ‘official’ national dish, I guess it comes from Foreign Secretary Robin Cook referring to it as “a true british national dish” in a speech

Chicken Tikka Massala is now a true British national dish, not only because it is the most popular, but because it is a perfect illustration of the way Britain absorbs and adapts external influences. Chicken Tikka is an Indian dish. The Massala sauce was added to satisfy the desire of British people to have their meat served in gravy. (full speech) (wikipedia)