With a series of hit titles like Friends, Joker, and upcoming big releases like Green Lantern, Doom Patrol, Zack Snyder's Justice League and Strange Adventures, HBO Max is coming out of the gate with plenty of reasons for people to sign up for it's newest premium streaming service. However, it appears that not everyone will have access to the service at once, and it may keep fans across the pond quite a bit longer than they may have anticipated to stream HBO Max.
Due to preexisting deals with Sky Atlantic (which was briefly owned by Disney following it's purchase from the now-nonexistent 20th Century Fox, but now is in the possesion of Comcast), HBO content will exclusively be available through Sky, meaning that the HBO Max service is currently legally unable to launch in the UK region.
This was confirmed earlier by a series of tweets from Sky's official customer service channel, confirming that HBO Max would be unable to launch anytime soon and that HBO Max Originals would also be unable to be streamed through Sky.
While it is possible HBO could make another deal in the future (like how they struck a deal with NBC to have all 8 original Harry Potter films streaming on launch day in the US), given the size and nature of the deal that does not seem likely at this time.