I think Costa does more normal teas and coffees rather than milkshakes or whatever you're referring to.
But then McDonalds, I think, already sell more coffee than anyone else in the UK, so I guess they're converging from both directions.
So when Starbucks came with a poor product the only people who went were curious locals (who didn't return) and Asian students who liked the brand:
(Omad = one meal a day)
Similarly, they also sell very-high-sugar cakes and drinks.