I heard England has 3 million Muslims, England has a total population of 53 million. So, that nears 5-6 percent of the total population that are Muslim. That doesn't look like much of a problem but then, people start looking at fertility rates, what if a native English family has 2 children and a Muslim family has 4 children. It's a lot to think about and how it affects the future say 40 years into the future and so on.
Well, yes, maybe.... But that's a purely static analysis, and it does not account for the fact that cultural changes can go in both directions.
The same dark warnings were out there in the US of back in the mid-19th century, when the Know-Nothing Party made a name for itself by warning about the great dangers posed by the millions of Irish and German Catholic immigrants who were flooding into the country. However, the Irish assimilated, as did the Italians, Poles, Germans, and all those others. It's basically impossible for us today, to understand why the Irish or German immigrants caused such fear. Why are they not a problem? Because they were expected -- required -- to assimilate, and they did.
The point is, it isn't so much a matter of how one is born, but rather the sort of citizen one becomes. A nation that expects and requires immigrants to conform to its basic cultural norms, will eventually assimilate the immigrants. For Islamic immigrants, I suspect that "assimilation" will naturally follow as soon as Islamic women are expected and required to become part of the normal culture of Britain, France, or Germany. Islamic men seem to be the problem right now -- but in any society it's the women who change the culture.