Good question! The younger generation of British Pakistanis are quite different from the older ones; living in a Pakistani occupied area for many years I know this well. Most of the older generation (those who started coming in the 1960s) are decent folk. I grew up with these people and thought nothing of racial differences. However, as families, they have tended not to mix, keeping in their own ghettoes, and in these ghettoes the younger generation have grown up, separate from whites, so that when whites and Pakistanis come together they clash. This is natural animal behaviour that you cannot socially engineer out.
The last 20 years you mention would be consistent with Tony Blair's and Jack Straw's mass immigration project. (Jack Straw was MP for Blackburn - he wanted the votes!) A new type of Pakistani muslim came in, and in great numbers. They didn't want to mix in or work for a living, they just wanted free stuff and felt entitled to it. They were more purely muslim, you started to see the more outrageous dress of the muslim world, robes and headgear - dress that you never saw before except maybe at weddings. You certainly never saw the burka until the last 20 years or so. You started to hear of muslim principles, such as not obeying any laws except Allah's.
In the Blair project you got other muslims as well, not just Pakistanis but Afghanis and Bangladeshis and the rest, men from cultures where women are covered up. If you want an idea of how young muslim males behave at the sight of free, Western women, look at what happened in Cologne on New Years Eve a few years ago.
This is why things have changed so in the last 20 years. Not all Pakistanis are the same. The younger ones are giving the older ones a bad name; I have known older Pakistanis move out of the area because there are - guess what? Too many Pakistanis!
