Assuming the business works the same in the UK as it does in the US, it simply isn't a priority to worry about the schedule 2-3 weeks out.
First, release dates change. Yes, this may sound surprising, but what is expected to be one of the year's biggest movies -- Avengers: Infinity War -- just changed its release date up one week due to the unexpected huge success of Black Panther. A series of changes followed, with Rampage moving its release date as well, and a handful of other changes to the schedule. Had I programmed my showtimes for a month out, I would have been frustrated by this. So, why do it?
Second, auditorium choice is not known. While it's usually pretty obvious that a movie like "Avengers" is going to get the big house, plus a few more (if you're a 20-screen, for example), it's not known how to distribute those other movies around. Do you move Black Panther down to a 60-seat house? Do you keep it in two 100-seat houses? With Star Wars opening, how many screens am I going to allocate for it at a given show time? Does Greatest Showman need a bigger theater? Do I need to space the showtimes out more to allow for larger crowds to efficiently access the concession stand?
Demand for the upcoming week is best gauged by looking at the immediately preceding weekend. This means the schedule gets finalized on Monday at the earliest - some theaters update their schedule as late as the day before, usually to add shows.
Major releases usually have a single auditorium posted a month in advance. You'll often see theaters put tickets on sale for the big movies - Star Wars, Avengers, etc. - this is done in anticipation that the movie will play in one or more of the biggest houses. These schedules can easily be set, because it's obvious that a Star Wars movie is going to be the biggest show of the weekend, and probably the weekend after. Studios actually schedule their competing product around this fact. So, it's easy to put these out weeks in advance.