To some extent it may depend of whereabouts you live in the country to how you regard the subject of integration, or at least how difficult it could be.
Here in West Yorkshire the local ethnic Pakistani community is largely drawn from a few areas of Pakistan, like Mirpur and Kashmir. The people from this area have lived in Yorkshire towns like Wakefield, Dewsbury, Bradford and Keighley since the 1950's. There hasn't been much integration in 60 years. The community is very clannish.
Third generation British Pakistanis still speak Urdu at home. First cousins regularly marry.
A few years ago on Look North an elderly Pakistani lady was interviewed. She'd been here since 1958. Didn't speak one word of English, had to have an interpreter.
In a domestic violence case a few years ago one Pakistani woman had been kept so isolated during her 17 year marriage she didn't even know what UK money looked like; her husband had kept her a virtual prisoner in the house.
In this community women are kept away from outsiders, certainly as far as friendship / dating is concerned. Not only is going out with a white boy unthinkable, going out with anyone from outside the local community is. One recently arrived Muslim asylum seeker from the middle east didn't realise this and asked a local Pakistani girl out. The next day he was beaten unconscious by the girl's male relatives.
I used to work with a couple of ethnic Pakistani men with no issues, in fact we were always friendly. But when one told me that if he had girl children they would not be allowed to marry a white man I couldn't help but reflect what would happen if I'd said the same thing in reverse.
So I don't think Mr Cameron realises the scale of the task. In other areas of the UK things might be easier, but around here he has his work cut out, and then some.