What's the worst coffee in the UK?

A lot of people don't understand the difference in brew ratios, the beans you start with, and the milk to coffee ratio make to a cup of coffee. The problem with all the franchises is that quality control of all those things is probably poor. So not only do the chains vary, but outlets within chains vary wildly. They are convenient though so I use them too even though, like you, I'm a keen home coffee nerd. But you'll always get far more bang for your buck with a good independent cafe / roaster where all the above mentioned variables are controlled and cared about. All that said, I'd have to say Costa seems to use a more medium / dark roast and is on average better. Try a cortado from then and without 4 gallons of milk, the coffee isn't too shabby. I had an espresso from Starbucks once, just to see for myself. It was rancid. Like the darkest, burnt, over extracted shot I've ever had in my life. It is for people whose idea of different coffee is the size of cup or flavour of syrup. Which is fine of course. You pay, enjoy what you like :)