I'd say that Trump Travel Ban, which did include Libya, would have been good advice for the UK.

If the immigrants to the UK aren't going to assimilate, then why bring them in the first place.

Salman Abedi had over 20 years to assimilate!!!!

The moderate Muslims in the UK, "just living ordinary lives", will likely remain "moderate" with respect to non-Muslims, until such a time as Muslim Hard Liners take control. Muslim takeover in the 21st Century is about Campaigns that last, not for years, but for Generations. When they reach that "critical mass" in the UK, with their birth rates, whenever that turns out to be, well, then it will be too late for the non-Muslims to even think about reversing course.