My home city of Dundee has a very low Asian population compared to other English towns of a comparative size, e.g. Coventry. The largest "ethnic" are Pakistani/Bangladeshi and it is they who own and work in the majority of takeaway and/or sit in "Indian" restaurants. It is therefore a curious misnomer, technically, to describe this food as "BIR". Back in the seventies I can remember takeaways having signs that said "Indo-Pak" cuisine.
I don't think I've ever seen anything other than white customers in these restaurants though, and therefore I assume (as is the conventional wisdom) that BIRs exist, in the main, for Caucasian and not Asian customers. This cuisine could then be assumed to be unique to the UK, and not reflective of the cuisine of the origin countries.
I wonder, if like a lot of things, BIRs grew throughout the UK from a starting point in London? I know that there is some evidence of curry houses like Veeraswamy serving food there in the early 1900s.