My current favourite place for coffee these days is Caffé Nero, in the main as I really enjoy the coffee. The environment is nice too., tables and chairs or comfy sofas depending on what you want or need to do. Good service too, making sure you get what you want and provided efficiently and quickly. I have had consistently good service from the different branches of Caffé Nero I have been to across the country. The coffee is great, but is quite strong, much stronger than a similar coffee at Starbucks or Costa. However you can ask for a weaker coffee if that is your preference.
Alas there are not as many Nero branches as they are of the other two, but I have managed to find them in most major cities.