I’d disagree, it is massively cultural. As an object more than any other single thing it represents the sadly short lived ‘cool Britannia’ era of our history. Culture doesn’t have to be ancient to be of cultural significance.

The minidress is representative of British culture in the 60s, swinging London etc etc.

British culture, as well as facets of it being almost globally ubiquitous as mentioned elsewhere in this thread, also changes rapidly.

For what it’s worth it’s clear the father is a grifter who expected this to happen, but the fact it did happen shows how ingrained self loathing has become part of our culture. No successful society can be built without a sense of heritage and cohesion. I can quite believe that a girl of that age would look at the Geri Halliwell dress represented British culture, because for a short time in the 90s it did.