Hi Suppster,
Yes, I suppose it is true to say that cooking (including BIR curry cooking) "evolves" with time.
It is also true to say that curries seem to vary from restaurant to restaurant, chef to chef and, particularly, from region to region.
However, there is a common "cooking ingredient" to a BIR curry; that being the use of a curry base and precooked meats. However, some people would even disagree with this.
But I would define a "BIR curry" to be those that are produced using a curry base (one main criteria amongst others). Though times change, I suppose, and more "traditional" type curries (i.e. those made without using a curry base) seem to be becoming more prevalent.
Similarly, some people (including me) believe that the BIR taste is primarily derived from their curry base. However, some people think it is derived primarily from the "techniques" they use to cook. The truth is probably that it is both, of course.
But certainly you would be correct in assuming that the "huge pot of gravy" is a typical feature of typical BIR curries.