Today, chilli powder in the UK, as sold by the major supermarkets and the largest independent brand, is a US/Mexican style blend of powdered red chillis with herbs, spices and seasonings such as oregano, garlic, cumin and salt. (Asda mild,hot; Sainsbury's mild, hot, Tesco mild, hot; Schwartz mild, hot.) Until I found this out, I'd always assumed that chilli powder was exactly what it says: powdered chilli.
It appears that at least some UK brands are just that – Morrison's don't list ingredients for their hot chilli powder and the web page for their mild chilli powder just says "Ingredients: chilli powder". The bag of chilli powder I got from an Indian grocer also lists no ingredients, so I assume all of these are just powdered chilli.
What is the history of this? Has chilli powder in the UK "always" been the US/Mexican style blend or is this a more recent phenomenon?
Please include evidence beyond personal recollection in any answers. Personal recollection is unreliable in this case, because everyone thinks they know what chilli powder is and most people mistakenly believe that it's just powdered chilli. This question is an attempt to clear up the confusion in the comments to an answer on "Confused about cayenne pepper, chili powder and paprika?".