I think the best way to put it divebuddy is that I'm not saying you're wrong, but that the statement of such-and-such an area being 100% Muslim requires a great deal of specifics to understand. How large are these areas, etc etc.
I'm fairly sure that somewhere not that far from where my parents live, Harehills in Leeds, has a huge proportion of ethnic minorities -- but even there, the density of Muslims is something like 50% of 64% of the 27,000 total residents, which is a far cry from 100%.* Maybe in Bradford.
*source:
http://www.volition.org.uk/wp-content/uploads/2013/02/2011-Census-BME-info-Ward-breakdown-Feb-2013.pdf , and making the assumption that everyone of Pakistani or Bangladeshi origin is a Muslim. Also
http://www.ukcensusdata.com/gipton-and-harehills-e05001424#sthash.D288nWq1.dpbs , that appears to confirm my guesstimates of about a third of the population.