Every 'culture', i.e. the way of life in a place at any given time, is made up of influences from other 'cultures', from travel, dispersal and migration, from the past, and as generations change. This applies to all the 'cultures' who influence one another.
People use narrative to connect themselves in the now to people who lived in the same place in the past, or who belong to the same tribes, broadly, 'ancestors'. This creates a sense of belonging, continuity and tradition, to counteract inevitable changes and disruptions. All cultures do this.
What is particularly important to people in the now is what we call living memory, which is our memories, partly composed of the individual and community memories of our parents and grandparents. In reality distant 'ancestors' wouldn't recognise our world, values or even our version of their languages. Living memory gives us an important sense of stability, family and community stability, and helps mediate change. For all societies, all cultures.
When this is disrupted suddenly, or change happens too quickly for community cohesion, it damages the existing society and fragments previously shared culture. If change happens more slowly, cultures will assimilate new elements, although distinctions and oppositional elements will always be part of a culture. All societies, all cultures.
A large nation-state will never have a monoculture, where it has shared cultural elements, laws etc. that's by agreement, the formalisation of the agreement is citizenship. In times of war, governments will try to appeal to a consensus version of a national culture to motivate people to defend it - the British government did this rather well in WWII, the cross-class culture on the propaganda posters was both real and not real.
In the UK, the establishment (education, policy-makers etc.) have spent the years since the end of Empire treading down rural cultures and working-class cultures, and in particular telling English people their flag alone shouldn't be seen. This seems to be a reaction by the educated English middle class against its own supposed origins, wanting a cultural narrative that aligns it with an internationalist Enlightenment, not colonial warmongers of the long dead upper classes. If they can find a 'Celtic' ancestor, so much the better.
So now Craig, white panel van man from Chesterfield, with his facebook profile St Geo cross, represents an embarrassing failure for the new enlightenment because he's apparently clinging to an imaginary Britsh culture of the past. It's your fault, Craig, just drink your Monster, eat your Biryani and shut up.