Metropolitan londoners move with culture at ease, and so do parisians, berliners, etc. But ask the opinion of a brit in benidorm and you will get similar confusion on local customs, a similar feeling that "the UK is the only place that exists, the rest of the world is a holiday spot". The provincial sort, people who live in small town or city, people who are not well travelled; their entire world view can be defined within a country.
And also, for the record, the school uniforms do still baffle me a bit. I tend to play UK apologist in the US and US apologist in the UK. So for full balance, the lack of electric kettles in the US also baffles me. Trust me, Brits who visit me in the US are just as vocal about their bafflement with the local customs as the inverse...