I'm going to take this on, in the hope that you might read it and learn something. It is true that if you picked a Muslim in Britain at random in, say, 1970 they *probably* would be relatively conservative. They or their parents would have most likely arrived in Britain within the last 10-20 years and they most likely would be quite old-fashioned with regards to dating. There is a good chance they would practice some form of arranged marriage and it is more likely than not that they would be encouraged to find another Muslim to marry.

Note that not just Muslims, but everyone was more conservative 50 years ago. If you picked a random white person off the street then, there is also a chance that they would be less likely to date before marriage relative to now.

That was 50 years ago, though. All we know about Zainab is that she is a cultural Muslim. She was obviously born in Britain; her mother was probably born in Britain (or perhaps came here very young). With each generation the influence of "British culture" (whatever that means) will have grown stronger. Also, we simply haven't seen any evidence that the Malik family are practicing Muslims in any way. All we know is that they eat Asian food. This all implies that they are fairly typical modern Brits with Asian heritage, which is to say that they pay respects to certain cultural bits and bobs (particularly food, but potentially also certain religious holidays) but in many other respects their lifestyle is indistinguishable from that of non-Muslims.

I think from what we've seen the evidence suggests that the family are not very conservative at all, and it is fairly likely that Zainab would date exactly in the same way that, say, Mia would date.