This is so baiting but I'll answer in good faith.
If you've lived/travelled around the UK you can notice differences in how the majority of people look. Eg there are many more red heads in the NE of Scotland, more olive skin in the south of England. More freckles in NW Scotland, longer faces and noses in the south.
Even height noticeably varies across the country.
So 'ethnicity' isn't just some shorthand for 'white' as you insinuate.
Personally having done genealogy the surnames of my great grandparents are traceable in the UK going back hundreds of years, some 1000.
Epigenetics is a new and interesting field that is showing that we carry the experiences of our ancestors down through the generations.
I feel a personal connection to physical events that have happened in the UK even before I was born eg the 2 world wars. I have a shared history, culture and values that aren't shared by most immigrants.
I welcome newcomers into my community but I believe that integration is crucial. Ghettos need to be discouraged. Why shouldn't we expect people to learn English and adhere to our laws such as not doing FGM?
I worry especially for women's rights that people who don't have the shared cultural history of the suffragettes and second wave feminism don't feel the need to follow our laws regarding women's safety. See Rochdale & Rotherham & other gang rape culture that starts with young men from other cultures seeing girls as objects.
I don't think people who have committed these crimes should be allowed to remain in the uk.