There were 9.4 million (legal) immigrants living in the UK in 2018, out of a population of 65 million

On the subject of 'nativism' (which means the wish to halt immigration and which many are intent on making a thought crime) here is some useful information.

In 2011 just under seven million people living in Britain were immigrants – enough to fill a city the size of London. In 2016 the total number of people living in the UK who were not born in the country, which includes those who became UK citizens, was 9.2 million. It rose to 9.4 million in 2017.