Why is Britain not an immigrant country?

The so-called immigrant countries are all countries with sparse population, where only a few indigenous tribes live, which were discovered by westerners during the great voyage, and then white people entered, colonized, developed and settled. For example, the United States, Canada, Australia and New Zealand are all such countries.

Britain is a traditional European empire and has always been a country with a long history, developed economy and dense population. Countries like this have limited ability to accept immigrants and have no subjective needs in this regard, so Britain is not a real immigrant country.