CL, you're assumption is that 619,344 Muslim women will need to produce offspring for 80 years. This is not true, as the offspring of these women will also produce in later years, as will their offspring, ...
The white UK population are in decline, with a birthrate below that required to sustain the population, while the Muslim population, with a birthrate of around 3, are multiplying rapidly.
Anyhow, it's not just about numbers, it's about the ages of the population as a whole, and pretty soon there will be a vast increase in the number of young Muslim VOTERS that will determine the direction taken by our government. Also, with lots of healthy (thanks, NHS) young Muslims able to put up a good fight, we will have a hard time defending ourselves against them.