Remove all immigrants and children of immigrants from the UK and it would have an extremely old population. As in median age of ~60+ in dire economic straits which would push everyone who could to abandon the country. Small boat arrivals are a trivial number of people irrelevant to this discussion.

You can’t have both negative population growth and 300,000+ people leaving the country every year for very long without massive issues. And those massive issues are going to push even more people to leave resulting in even more issues. Which would effectively force the country to promote immigration even if it doesn’t want immigrants the UK needs them.

Alternatively, if the population growth rate was positive then the country wouldn’t have nearly the need for immigrants to be stable.