I'm not sure I believe that totally. Manchester around which the Lancashire milltowns circle is vibrant economically, Leeds in Yorkshire is a dynamic city. Its not the days of George Orwell writing The Road to Wigan Pier.
Its bothering me alot, because the entrenchment and self segregation seen up north is much worse than elsewhere. The midlands has Pakistani / Muslim ghettoes, but at least as far as I can tell, this is not happening there. Some other dysfunction is at play.
I think that the many factors and issues that have been pointed out, come together to form a perfect storm in the particular social context of these small milltowns (including the lack of an economy vibrant enough to force people out of their segregated lives), factors that are mitigated or nullified or adjusted elsewhere.
Like you said, it's complex. There may not be one overriding factor at play here but a culmination of various ones, however I do think Straw could have chosen his words a bit more carefully when analysing the issue. I'm not saying he thinks that there was predominantly a racial or ideological motivation to the sexual abuse of these girls but someone reading his comments could have interpreted it that way.
I see the situation like this - given the troubled backgrounds of these girls, these sexually frustrated men (with no self control) saw them and thought it would be easy to lure them and in their mind justified their actions as a minor wrong, if an immoral action at all because they probably saw their victims as 'immoral' unchaste women.
This maybe an over-simplification of the situation but I think the contributing factor was the fact these girls were vulnerable.