I'm a Londoner of an Indian origin, and a massive foody.
Reading this thread, it seems there are huge misconception about Indian food in the UK. Chicken Tikka Masala is a national dish of the UK.
India is a huge country and food changes from state to state, and from city to city. It is very local, Gujarati food is very different from Rajasthani.. and so on. Large proportion of India is vegetarian, thus you may not find meat samosa everywhere, but it is very much Indian. Again, even samosa recipe differs from state to another state in India.
First thing, using powder masala is NOT main ingredients of Indian restaurant dishes in UK anymore. Indian ingredients are now widely available on every high street in the UK.
Indian food in the UK has evolved to local taste, and we now have fair few amazing Michelin Star Indian restaurants in London which are absolutely amazing. And chefs like Atul Kocchar, Vineet Bhatia, Reza Mahammad, Vivek Singh, Rohit Ghai are doing amazing stuff. We now have many top restaurants who experiments with Indian flavours with amazing result.
So how does it differ? Take Chicken Tikka Masala, this dish was evolved in the UK and now a national dish of the country, and you won't find this in India. Madras and vindaloo are not Indian, Indian food is not supposed to be "chilli hot", it's delicate balance of spices. Many Indian high street take away restaurants in the UK are run by Bangladeshi, thus taste will differ to authentic Indian food. Sadly, may high street Indian restaurants have one basic sauce and thus many dishes taste alike, and far from authentic. Most high street restaurants have few of same dishes.
So, Indian food in India will differ vastly, it will not be creamy that you may be used to. Not every restaurant in India will have a tandoor, thus naan bread will differ, roti is more norm. Vegetable dishes will have more flavours in India. Sadly, I always find people in India use far too much oil and ghee than I like.