The fact that three million+ U.S. citizens visit Britain every year, without apparently encountering much in the way of serious problems, and that these numbers don't seem to be declining, suggests that your father's opinion isn't widely shared.
There are two parts of any country that gets a lot of commerce with tourists from abroad. There’s the part that the tourists see, and the part that’s out of their sight.
Sure, three million + Americans get to see the Changing of the Guard, The Tower, and John Lennon’s birthplace, or perhaps even the house that ST Coleridge and his family occupied in the Lake District, and whatnot. Those are places designed to look nice so foreigners will come and spend money. But that does not mean that the less traveled places in London and England are just as hospitable and just as safe. News only happens on TV in the US, but not elsewhere in the world, where they know that the official TV version of events is bogus fantasy, because they have to live with the reality that only sometimes gets noticed in TV-land.