I can't speak for how the UK premiere is being handled, but with the U.S. May 5th screenings, there's a "global review embargo" that's lifted on Saturday, May 6 @ 8:45PM PST / 11:45PM EST.
Embargoes tend to be handled differently depending on the studio or movie. Please keep in mind that while there are press, talent and industry insiders invited to these things, they're usually allowed a "plus one guest," so you DO get non-professionals at these things. I definitely know a person who is going to the UK premiere as a guest. Sometimes invites clearly state that even a guest must abide by the embargo. I've been to screenings where you sometimes even have to sign an embargo waiver, sometimes even the guest too. Other times, there's nothing stating what your guest can and can't say on social media, and I honestly have no idea if they even have the manpower to do anything about a random guest posting a Tweet.