I take issue with the word "immigrant" as it implies compliance, but okay.
No, that was not my intent, but I see your point. I think this is really all I meant to say:
Most former colonies of Britain can feel the influence of its culture a lot more than Britain feels the influence of any of its colonies' cultures.
So when Britain says "we totally invented how to put butter and spices in a tomatoe base and add some chicken". And tries to claim one of the last few things they haven't from this subcontinent? I get kinda angry.
Who is this mythical 'Britain' that is saying this? Is it the King? Is it Farage? Or is it the 8% of our population with South Asian heritage? We're a multi-cultural nation. The colonial and imperial roots of that absolutely can't be ignored, but they don't erase the stake that BAME people have in this country.
Some British people originally came from the sub-continent; get over it
It was invented by a British person in Scotland and is beloved throughout the UK. Why wouldn't it be the national dish of Britain? The man who invented it, Ali Ahmed Aslam, was born in Pakistan, but had lived in Scotland since he was a small child, meaning he grew up steeped in British culture.
I don't think a cuisine has to be based on cooking that originates in a different country to be a country's national dish. He decided to improvise a sauce that was not especially South Asian considering it contained a New World fruit in his restaurant in Glasgow and that inspiration became the favorite food of millions in Britain.