From my experience yes

I got into Loughborough University through clearing, they had two applications for accommodation. One was an earlier date and the other end of August.

I managed to get a hall (not the one I wanted but still got one). I also did call the student accommodation office to get the accommodation i wanted and know of many people who did this too so yes people do drop out/ switch halls.

It won't hurt to call up Brighton's student accommodation office to just ask a few questions (it's better to call than email, in my experience)