Last time I was in the UK, nearly everywhere that sold soda offered the full calorie, full sugar version of Coke. Pepsi and other drinks were often only available in lower or zero sugar, but Coke was almost always available in the full sugar version, for some reason.
Pepsi and Dr. Pepper decided to go lower sugar with supplemental artificial sweeteners to keep their price down, whereas Coca-Cola kept the original formula, but it costs more.
To put figures to this:
- Pepsi (11g sugar per 250 ml): 8.8p/100ml
- Dr. Pepper (11g sugar per 250 ml): 10.0p/100ml
- Coca-Cola (27g sugar per 250 ml): 14.2p/100ml
"full sugar" Dr. Pepper also contains: Aspartame, Acesulfame K
"full sugar" Pepsi contains: Sucralose, Acesulfame K
So after the sugar tax some people moved to these hybrid drinks whereas others just moved whole-hog to Pepsi Max and Diet Dr. Pepper which are commonly cheaper and have the same aftertaste as their "full sugar" variants.