Razor
As far as I understand that is the case, and if you check out Bruce Edwards' article Curry House Cookery, you'll get a bit more info. Some BIR Vindaloos add in potato but that's not do do with the spicing.
I don't know what other members think but I've found that Scottish BIRs use more spicing than English ones, so for example our Madras is stronger, i.e. nearer to Vindaloo level of spicing than you might find in England.
I think, like all cookery, even BIR has regional variations and customs.