Even if there was no immigration, our culture would still be changing at a rate of knots. The culture you love was born of the foreign influences it had already experienced, particularly since colonialism, which is both considered the height of British culture from which we are descending, and the days which brought out the worst in us.
With increased travel and communication, cultural contamination is now entirely unstoppable. Cultures are spreading around the world and being deliberately exported, the distinct cultures within our country are also blending and becoming homogenised. But at the same time, cultures do remain ingrained in our minds to be associated with different regions, and new cultures are emerging, of blends of distant cultures and born of common experiences. People aren't just losing their cultural identity, they are also forging their own. This process is only going to speed up, and as much as you may love what has past, new generations will decide they like it the way was now, and later ones how it will be after this, and so on. That will be considered the best, and that any deviation in any direction is degradation. They too will try and fail to prevent change, not realising that change is the only source of cultural vitality.
There has never been a "British culture" to preserve. It has always been varied and different throughout the country, and no particular state of it has ever lasted more than a few years. You're fighting a futile battle to protect an imaginary past.
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