Then your assuming that the 12% of Muslim population in London is all male.
Even if only 3% is Mohammed then that's still a lot more than any other name.
There are more than 100 popular names in London, especially when you consider how multicultured the population is. So most popular names will be 1% or less.
But no other culture uses a single first name as much as Islam uses Mohammed. Whatever the statistics are it's no surprise that it would be the most common.