I was reading about Churchill and it struck me that his favourite whiskey was apparently Johnnie Walker Red Label. This seems like a shockingly low end tipple for an extremely posh Englishman of his era (I'm personally not prejudiced against blended whiskey- I love Black Label but the Red tastes like turpentine).
A bit more casual googling led me to find that Dewars and Chivas Regal were patronised by Queen Victoria. Again, while not bottom shelf, these aren't exactly prestige brands by modern standards.
Exhaustive reading of PG Wodehouse research into Edwardian England seems to indicate that brandy and gin were perhaps more in favour (at least with Bertie Wooster).
Was single malt just not widely available? Or was whiskey mainly served as a highball or in cocktails?
Single malt whisky wasn't popular until the 1960s and later.
In considering why, it is important to remember why blended products generally exist. This isn't just true of whisky but is also true of wine, tea and coffee amongst others. Effectively, by blending you are taking a bunch of potentially very variable input ingredients (often with many of individually poor quality) and producing a consistent and potentially more palatable product than the individual inputs would have been by themselves. For lots of consumers even today it is more important to buy a product and know it will be the same (and hopefully good) every time than it is to get the variety and distinctness that come from single origin products.
Over the last few decades there has been a general move away from blended products as people tend to have the ability and desire to source a greater variety of distinctive single origin products. In a broader sense, people in the present also just have more of an interest in provenance and the "authenticity" of single origin products than they did in the past. There is also the fact that due to modern production techniques it has been easier to ensure consistency in single origin products without blending. This isn't just a point about whisky, as I said before just look at speciality coffee as another example of the same phenomenon at work albeit on a decades later time schedule.
Now getting more specific on whisky, it's worth noting it wasn't considered high class at all until after phylloxera severely damaged the wine crop in Europe from the 1850s-1870s. Before then men of class in England (and elsewhere but the English market was the most important thing in propelling whisky to prominence) would have drunk brandy (if they drank spirits), and whisky would have been viewed as a curiosity at best or a sort of Highland moonshine at worst. But as the base ingredient of brandy (wine) became much more expensive due to phylloxera, people switched increasingly to whisky effectively out of price necessity.
So bearing that in mind, that whisky went fairly quickly from not being classy at all to being a respectable drink, one can see that single malt whisky may not have cut it. It took blenders to craft the inconsistent, often poor quality and disparate products of individual distilleries into something the respectable classes wanted to drink.
By doing so, blenders could also market themselves as a respectable, safe (as a side note the period when whisky became popular also coincided with the period where food safety and adulteration started to become a major concern) and consistent product which single distilleries couldn't do.