Weekly leave is allocated in advanced.
You get 9 daily leave days every year (rising to 13 after 15 years) where you can apply to book any day off Mon-Sat.
Sundays are optional overtime, they will appear in your link but you can have them all off if you give 3 or more days notice.
At the moment every Saturday is off due to strikes and this has been the case since last September with no end in sight.
So at the moment you can have every weekend off.
Weekly Leave is allocated in advanced, of which can be changed by moving it to vacant weeks (which there aren't many of in Summer) or swapping with someone. However a new starter might be allowed to move their weekly leave to where they like for the first year, even to weeks which don't have any vacancy. I'm not sure if its true but I think I've heard this mentioned before.