https://www.pewresearch.org/religion/2017/11/29/europes-grow...
The most recent birth rate stats I can find is 2005-2010 where Muslims have a birth rate of 3.0 while the rest of the country is at 1.8. Estimates say it's more like 2.5 now, but the current overall UK birth rate is only 1.57 from a quick Google.
https://assets.pewresearch.org/wp-content/uploads/sites/11/2...
The same article also states that the average age for Muslims is much lower than non-Muslims as of 2016, 28 vs 41 for the UK.
https://www.pewresearch.org/wp-content/uploads/sites/7/2017/...
It is impossible for 2 typical wage earners to pair off and start a family.
https://en.wikipedia.org/wiki/Income_and_fertility