This series has just been on TV in Australia, missed the first couple but the Sri Lankan episode was great, and this week he was in Bangla Desh and travelled from Dhaka to Sylhet where he reckons most of the families came from who eventually set up the BIR restaurants in the 50s and 60s.
We saw a few dishes being prepared, the Biryani was brilliant looking although no chilli of any description. A few of the others looked a bit "meh" to me. For example the ladies doing a big pot of ground shallots, Garlic and Ginger, tomatoes, some turmeric, cumin, salt and that's your base. now chuck in a heap of potatoes and some fish pieces, some coriander leaves and a few green chillis and simmer it.
Struck me as the sort of curry I would make years ago when I only used Turmeric, Cummin, Coriander and Chilli and everything turned out a shadow of BIR (or AIR).
Later at Sylhet he bumped into several English guys of Bangladeshi descent who were back on holiday to visit the rellies and they all reckoned that Bangla Deshi food is rubbish and BIR is the best

Certainly an eye opener.
Edit: when I worked in Brisbane in a part of town called Fortitude Valley which comprises Chinatown, but quite a lot of Indian and every other ethnic group gravitates there as well, there were 2 Indian lunchtime cafes right next to each other. One served "real" Indian food - usually 3 vegetarians such as peas and panir, lovely rich curries that only had potatoes and pumpkins as a base, variations on "Chole" etc. with a magnificent veg biryani on offer most days - plus Dhosa and puri to blow you away.
Right next door was a Lamb Madras, Butter Chicken and Beef Vindaloo joint. The latter was always full of Indian Students, taxi drivers etc and while the "genuine" place got its fair share of business the Indians and Bangladeshis seemed quite at home eating the AIR offerings. Wonder if there's a bit of a folklore myth happening about how BIR / AIR is something that most people in the Sub Continent would sneer at.