Chilli powder in the UK
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