Why aren't UK cinema listings available more than a week in advance?

I'd assume that most cinema visits are fairly "spur of the moment" decisions, often not planned more than a day or so in advance. Yes, there are exceptions, e.g. people wanting to view the latest installment of a popular franchise on opening day. So there's little point in posting "definite" schedules weeks in advance.

Moreover, such strict schedules would interfere with a movie unexpectedly becoming a hit: there's a rush of visitors but the cinema cannot accommodate them due to an arbitrarily compiled schedule weeks ago. Meanwhile another movie is under-performing and that same cinema ends up with nearly empty theaters. Thus they need wiggle room and must be able to shift movies to smaller or bigger theaters depending on demand, or change schedules to allow for more showings etc.

In multi-screen cinemas, it is hard to predict long term demand for each movie

The core reason why cinemas don't list movies a long time in advance in the UK (except for the weeks movies are released) is that future demand is hard to predict and cinemas like to have the flexibility to adjust their viewings to the movies with the largest demand.

Nobody knows weeks in advance what the customers want to see (which is rule number one in the industry at least among William Goldman's readers). If they did we wouldn't get catastrophes like Mars Needs Moms or John Carter. We can tell when the first week is (and therefore announce it in advance) but after that, the showings will depend on how many people still want to see the movie. A disastrous first week will see the movie with drawn and something else put in its place the following week. A lower than expected turnout for a major release might see the movie reduced to one or two screens (it might launch in 5 or 6 screens in a big multiscreen cinema if the expectation is it will be a very popular blockbuster like Star Wars: the Last Jedi.)

But in an environment that is fairly unpredictable, cinemas will make more money if they have the flexibility to use their screens for the movies that have proved to have the largest demand. In a 15-screen multiplex there will be a mix of large and small screens. Movies that have demand but not blockbuster demand will move to small screens. Movies with too little demand last week will be dropped altogether.

This is much easier to do than it used to be now that most distribution is digital. The costs of adding an extra screen are small (you don't need an extra film print just an extra hard drive). The investment to get the movie on many screens to start with is low leading to much lower regret costs per screening (if you have a lot of expensive reels of film there is more incentive to keep plugging the movies to pay off the distribution costs).

In short, theatres look at the popularity of a movie on release and then decide what will be screened the next week to maximise their revenue. So they can't usually publish their full screening schedule a long time in advance.

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.

Money. It's not UK thing. It's a European thing (where cinemas are independent from production companies).
There is a finite amount of copies that are available. And cinemas have to wait for their copies to arrive and distribute through their screens. And because on the weekends they have the biggest attendance they need to fix biggest income they can get (because on the first week only 10% goes to cinema pocket, rest go to distributor) with the possibility to do so.

And example. Valentine days 2018 fall on Wednesday. What is premiering on "valentine day"? New Gray movie. Does he really premier on valentine like he is advertised? No, they premiere it week before because attendance on the next week will be probably close to nothing. BUT, no we have opening weekend (that give a lot of money to distributor) and we have Valentines day that may earn a lot of money for the cinema (in popcorn money).

So then the cinema decide if they want to go screening Gray some more (with possible low income while hogging the screen) or want to show something new that will give them more profit (because the new movie will bring popcorn money).

So why they decide on Thursday? Because it's the day they can be sure they will have hard copy on their hands so they can schedule showing the movie. Yes, in the age of streaming, the digital movies are distributed on hard drives with seals so they will not leak. And the hard drives sometimes don't work, sometimes are broken, sometimes instead of 15 there are only 12 of them. And for cinema it's the worst case that they announce screening, pople come and the movie is not there.