Genocide definition:
the deliberate killing or severe mistreatment of a large number of people from a particular national or ethnic group with the aim of destroying that nation or group
Are people being put in British concentration camps for being British? No.
What country is invading Britain? Is Sudan invading Britain? Because that's a horrible military strategy; Britian is very far away from Sudan and Sudan doesn't have the recourses to perform such an invasion.
Is it whites being less common? Yes; but this isn't genocide. It just means fertility rates among white people are down.
If you are a white British person and you want whites to stay the majority in Europe, then find a white woman and have a lot of white kids that you can afford to raise.