So here´s an interesting bit of history. Although Indian "curries" were popular in the UK in the 19th century, the origin of the "Curry House" began in the 60s and 70s, and they were not started by Indian nationals, but rather by Bangladeshis. Today, over 70% of all Curry Houses in the UK are Bangladeshi-owned.
One extremely bright guy decided to standardize the menu. He offered hot (vindaloo), medium (Madras) and mild (Korma); chicken, beef, prawns. The sauces were all prepared and had simple "additions" to make what ever dish you wanted: vindaloo, dhansak, jalfrezi, patia, bhoona..the paradox is that they were "Indian" dishes prepared by Bangladeshis. When I was young(er), no Friday or Saturday night was complete without a "curry" after a skinful in the pub (which closed at 10.30 or 11pm).
The only place I knew in London in the 70s which served authentic (South) Indian food was a place in the Strand. It wasn´t until 2001, when Chutney Mary opened in Chelsea, that real, authentic Indian cuisine became available.
And the other odd fact is that Indian markets make loads of curry powder - for export. No self-respecting Indian cook would use curry powder!