I'm convinced that its success is simply because it's EVERYWHERE
I think this is it. I once saw a programme that tried to explain why Starbucks would open two branches on the same street, it was so they were always the nearest and therefore always got the business.
Plus their (Costa, Starbucks etc etc) huge buying power gets them the economies of scale.
A lot of the products may seem expensive for what they are £3 for a coffee, but when you think about it, they have to sell an awful lot to make up for the cost of rent, rate, wages, banking charges, fitting premises out, ingredients etc etc.
As for quality, there's an awful lot of style over substance, but they have the image/branding and for the product to be just good enough to appeal to the masses. After all, there's loads of huge brands making a fortune out of selling mediocre products, Subway, McDonalds, KFC, Burger King, Costa, Starbucks, Pizza Hut, Dominos, Bella Italia, etc etc.
People say they like the consistency, you know what you're getting, so most people must prefer to get something a bit crap, rather than go to an independent, where there is a chance that you'll get something far better than a major chain, but there is a risk of getting something really crap, like the Nescafe coffee and cheap bacon butty, but you can usually spot these by the lack of an espresso machine, decor of the place, and the pricing.