London — When people on Folkestone Street talk about mysterious outsiders, they are usually referring to the yuppie families who are attracted to its long rows of Victorian townhouses with steep walk-up steps, considered real-estate bargains by some. Here on the streets of Walthamstow, in the working-class suburban streets of northeast London, the Pakistani families who arrived in the 1950s are long-standing and accepted members of the community. It is a place where cockney pubs sit next to halal butchers, the poor whites and conservative Muslims making awkward but generally peaceful neighbours. But when dozens of police burst into houses...