Pret a Manger, or Pret as the locals call it is a chain of fresh gourmet sandwiches in the UK (and in the US now maybe?). It isn't the cheapest but it is fast, fresh and tasty. There are plenty around so you won't have difficulty finding one. It's like the Starbucks of Sandwiches. And speaking of Coffee the Pret coffee is good, although this is an acquired taste. I like it, some friends don't.
There is another chain similar to Pret called EAT but I don't enjoy their food as much. Don't get me wrong, it's still good, just not quite as good.