If hundreds of countries in the world fly the national flag, why do left wing UK people say it is bad for migrants?

I live in the UK.
As a foreigner (Swiss-Liechtensteiner), I don't feel outraged when I see the British or English flags up.
We are in the British soil, and I think it's normal to have UK/English flags on the street.

On the contrary, our national airline is excellent for migrants. Much safer than small boats.