So let's look at the dictionary definition of colonisation:
"Colonisation refers to the act or process of establishing control over a foreign area or people, often by settling in that area, exploiting its resources, and imposing political, economic, or cultural authority."
So it is factually correct to that there are certain areas of the UK where migrant groups have settled and indeed exerted political control (elected councillors, MPs etc) and cultural authority as well as dominating the local economy.
Examples include: Bradford West, Birmingham Hall Green, Birmingham Ladywood, East Ham, Bethnal Green, Blackburn, Bradford East.
So although many may dislike or find the term offensive or racist, by the dictionary definition these areas have indeed been "colonised ".