You just do one application. Wtf.
That isn't the case. You can do a partial application and then add more later. So, I didn't know this either. Apparently, what some schools suggest is to apply to a couple of "aspirational" universities, and use the response to that to steer the other three applications up or down.
It's hard to see why this is seen as being a good idea: by and large universities (contrary to the MN received wisdom) are not making nuanced decisions based on subtle criteria, so if you apply to five universities at about the same sort of tariff you will get either zero or five offers because you either have appropriate predicated grades etc or you don't. Applying for five places isn't like buying five lottery tickets: they will all apply roughly the same process to your application.
It is worth applying to five places even if you are certain of five offers if you aren't sure which you want to go to.
But a lot of students could perfectly reasonably decide to go to place X, apply to place X, be sure of an offer, and then use clearing if it goes wrong, thus saving a fair amount of messing about.
This is roughly what some of the private schools do for students for whom A*AA-ish is a pretty safe bet. They apply to Oxford/Cambridge as 1, UCL/Durham/Bristol/etc as 2, that's it.
They're sure of an offer from 2 (which will often be higher than the Oxbridge offer, particularly in humanities), and will do the interview for 1.
They then either firm Oxford/etc if they get an offer from them, or firm UCL/etc if they don't. They take your A Levels (which they are confident about) pretty much knowing where they're going.
If it all goes horribly wrong, they take a gap year so they can re-apply in September with their almost-but-not-quite-Bristol A Levels and go to QM, Kings, Exeter, etc who will give them an immediate unconditional offer in October.
This means they are always at the head of the queue for accommodation as they only ever go to firm offers, don't mess about going to lots of open days and applicant visitor days, take their A Levels pretty much knowing what's happening, and have a pretty safe plan B (i guess you could add "or clearing" into the mix, except that doesn't have the accommodation advantage and not all courses are in clearing).