Rapid and extreme demographic change due to government policy is a valid issue of discussion.
Obvious question. Is this change one that the citizens of that nation want, or will it be to their benefit?
Firstly do you have any objective evidence of "rapid and extreme" demographic change in the UK due to government policy (I'd argue if we are undergoing such change it is due to a lack of government policy)?
Secondly this country (UK) is a nation of immigrants Celts, Romans, Irish, Saxons, Normans, French, Dutch, Germans, Poles, Indians, etc. Believe it or not, you can find pamphlets (the equivalent of the newspapers of the time) in the 16th century, ranting on about the infux of French, Spanish, and Dutch who are coming here to take jobs from true Englishmen. Historically, this country has always railed about immigration, often with accompanying violence towards the imigrants but in due course things settle down and
we learn to get along together.