Others have commented above authoritatively on experience of London comps. I can give OP the lowdown on the London comp (‘non-selective’) application process, having just gone through it.
From mid-September - mid October my Y6 DS spent virtually every Saturday plus a couple of weekday mornings (school gave permission) sitting aptitude tests of some kind or other, total of 6 I think including the dreaded Wandsworth test (another story). All (except the WT) were 30 - 40 minute affairs so finished in an hour including registration and explanation. My DS met classmates on every occasion and being a sociable chap enjoyed being part of a small gang - almost a group playdate! Of course it was pretty much the same kids every time; yep, the 3 or 4 white-middle-class ones from his class. These are very much a minority at our state primary, though a growing one, due to changing local demographic mentioned by earlier poster.
They were being tested (apart from the WT) for aptitude in one of the following: Music, Languages, Technology, Art, Sport, at one or other of several oversubscribed comps more or less within commuting distance. Comprehensives generally have option of selecting I believe 10% of annual intake based on aptitude in one of these areas, originally to enable them to establish a speciality in that area. But it only works if the school is oversubscribed to begin with! Now I think they use it mainly as a means of ensuring they have a solid intake of able kids. Generally speaking, with exception of sport perhaps, it will be the same kids who do well in whatever speciality the schools choose to test for!
So, parents get the results of all these tests (including the WT) a few days before their completed CAP or London Schools Application form is due (end Oct), and they will, indeed are expected, to adjust the form accordingly, prioritising the schools at which their DC did well and abandoning those for which they now recognise they have no chance - for most likely they live way outside the tiny catchment area. That’s why, probably, they went for the test in first place.
Many DC, in fact most, will not qualify for any aptitude place. Many, if not most, won’t have gone in for any (it takes foresight and ruthless scheduling). Many, if not most, London comprehensives do not use them at all (must make for extra admin) but just stick to basic distance criteria.
But you can guess which schools tend to end up a magnet for the, um, entitled, with, um ‘gifted’ (or maybe tutored) kids. And, speaking from experience, it is very hard to resist any school which appears to have selected your DC ahead of maybe 100s of others. But that does not necessarily mean that it is the best school for him or her.
However, here we are. My DS, a week ahead of Offers day, appears well positioned for 3 aptitude places, and guess what, we put those schools in positions 1, 2, and 3 out of 6 on the form. In all probability at least one of them will come through for us.
But all 3 of them would mean a 30-45 minute journey. School 4 on our list would be a 15 minute bus trip and school 5 - a brilliant school in its very unassuming way - a 7-minute walk. School 6? 5 min walk.
Did we do the right thing, carting DS round to those popular but relatively distant comps when we had Outstanding but extremely socially challenged ones right on our doorstep? How does this give us any moral high ground over grammar and private school parents who send DCs across London to avoid their “dreadful” local schools?
To be clear, all inner London comps are socially (and financially) challenged. But yep, some more so than others . . .
Most of DCs class didn’t do a single test and will attend those local schools and I hope do very well, many kids do. And if my DS ends up miserable at comp 1, 2 or 3 he’ll be back on waiting list for comps 4, 5 and 6, and, seeing as they are less popular, could have that option later. But for better or worse I don’t anticipate that. Like I mentioned before he’s an able, uncomplicated chap who will befriend anyone, adapt to most things, without prejudice.