It's not a tough concept to grasp. The indigenous peoples of this nation are white and speak English as a first language. Such people make up 87% of the country's population. That they should now be in a significant minority in their nation's capital is an extraordinary thing. That such change has occurred in just 60 or 70 years is mind-blowing. The tendency to try and dismiss people who express concern or disquiet at such staggering demographic change as racist or antiquated is dishonest. Cleese is correct. London is no longer predominantly inhabited by indigenous (ie white and speaking English as a first language) people. Thus, London is no longer an English city.
Being home to foreign settlers and them and their children being in the overwhelming majority are two very different things.
And this country's 'melting pot' history is largely balls, btw. Up until the mid-20th century, the ethnic make-up of this country was massively homogeneous.