Amex has done a great job getting more acceptance in the UK (even converted M&S last year).

In contrast to above posts one rule of thumb I use when holidaying in France or Italy when I am looking for a good but not luxury meal is to avoid any restaurant or area of restaurants where Amex or DC are accepted. These tend to be tourist traps and I prefer to go to places where they look at you strangely if you produce the Green Card. The food and service is more authentic.