Chilli powder in the UK 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, as used for seasoning chilli con carne. (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&nbsp;&ndash; 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?".
There's a few questions you've asked in your answer, some explicitly, and some implicitly, so I've summarised the questions as I see them.

1) Is Chilli powder just powdered chillies, or a blend of other spices to create the dish "chilli"?

Both. As you've seen in your experience, this is a case of overloaded etymology, where confusion arises from the common name of the Capsicum genus of berries: chilli (or chili) peppers, and the mexican dish "chili".   

As for which came first seems to be a bit of a chicken-and-egg problem, as some sources say the dish is called chili because it was made with chili peppers, and some say they're called chilli peppers as they're the main component of the dish. Couldn't find a verifiable source for a definitive answer on this one.

2) When was the spice blend variant Chilli Powder invented (globally)

A cursory internet search leads to the article "The man who invented chili powder"[1], which covers the invention of the spice blend.   


  The family further claims that DeWitt invented chili powder, the now ubiquitous blend of ground chile pods and spices used to season chili, which may very well be true. It was certainly invented in Texas. Chili historian Joe Cooper credits William Gebhardt, a German immigrant, with first pulverizing dried chiles in a meat grinder in New Braunfels in 1896. But Fort Worth was electrified more than a decade earlier, and so, probably thanks to a mechanized chile chopper, DeWitt was already marketing a chili blend called Chiltomaline in local newspapers by 1890.


Cultures have been drying and grinding herbs and spices for millenia, so "powdered chillies" is nothing new, and indeed, would be my preferred distinction for referring to it rather than the blend.

3) When did the spice blend come to the UK?

As the blend was invented around 1890, at some point after that when someone thought it was a good idea to import it. Regarding it's stocking as such in UK supermarkets, there's not likely to have been a shift in one vs the other at any given point as the confusion will probably have existed since 1890.

4) How do I know which one is which?

The only way is to look at the ingredient list, which as you say will either just list chilli (although usually not the particular variant of chilli - or chillies, which is probably quite common to get a more complex flavour profile), or the other spices which have been blended. The Morrisons link you've given actually says in the description "pure chilli powder", corroborating the suggestion that it's not a spice blend.

[1] https://penderys.com/news/d-magazine-july-2018