OP, my DS is a day boy at a school with boarding choristers, and he recently went through the chorister audition process himself (unsuccessfully, as it happens).
From our experience, the informal audition is essentially a "qualifying" process, designed to identify the children with the vocal and musical aptitude to be serious contenders for a choristership. It is "informal" in the sense that it is usually taken by one person, such as the Director of Music, and it is not competitive.
If the response from the informal audition is positive, the child is asked to attend a formal audition, usually run by a panel of people. The formal audition is a chance to hear the candidate again to check that their performance is consistent, and to compare candidates if there are more applicants than places. Formal auditions usually also incorporate some paper-and-pencil academic tests to make sure that a candidate is at a reasonable academic standard - chorister schedules are very full, and there isn't the scope for much "catch-up" time for those who struggle academically.
For my DS, the actual content of the informal and formal auditions was the same. He had to sing a song of his choosing, play a piece on his instrument, read a verse from a hymnbook, do some vocal exercises to see what his range was, and do some simple aural tests to see what his ear was like, such as clapping a rhythm played on the piano, and singing individual notes from a chord.
In terms of timing, as far as I'm aware candidates are only seen in the academic year prior to intended entry (so some time in Year 3 if intending to enter at Year 4). Some choirs run informal audition days, others arrange them on an ad hoc basis as potential candidates approach them.
I don't think there is anything you need to or even should do in terms of formal preparation. Choirmasters are generally looking at aptitude (potential) rather than technical ability, and my impression is that they prefer untrained candidates as they don't then have to spend time changing already-learned technique or habits to conform with the particular choir's style. So there is no requirement for singing lessons to prepare for an informal audition.
It is a good idea for a child to have some experience singing in a choir, to see if they enjoy it. Many choirs also run "Be a chorister for a day" type experiences that give some insight into what choristers actually do.
Form your other thread asking about travel to prep schools, I assume you're talking about the major central London choirs. Bear in mind that some have specific entry requirements - Westminster Cathedral, for example, requires choristers to be practising Catholics (complete with references from parish priest). AFAIK the CofE cathedrals don't have the same requirements.