How often is "national pride" just shorthand for "our country is better than everyone else's"? That way lies racism, nationalism and, all too often, fascism. It has happened before, it has happened elsewhere, and there are people who would like to see it happen here.
You can't have, rationally, a blanket pride in your country. You can be proud of its beauty and proud of people's achievements, individually and collectively. You can be proud of the things you believe your country represents. But how can you be proud of everything relating to your country?
That leads to people insisting that everything belonging to their country is the best, and that neither they nor their ancestors ever did anything wrong. That's blatantly untrue, for all nations, yet you have plenty of people determined to justify heinous events in their country's history.
Do we want to be people who never question things about Britain that we know are wrong? Who never examine our own history? Who never challenge our government when they are mistaken, incompetent or downright callous? If we love our country, we should want to make it better, not go around breast-beating about how fabulous we are.