Just to give an honest low down on how things work from the perspective of an independent SW London prep (albeit not one with any blazers - or badges for that matter)! 
The school my dcs are / were in is co-ed and non-selective. Not known for being academically pushy, but did get them focused for 11 plus exams from around January in Year 5. All parents have a meeting with the head around this time to discuss options for possible future schools. The head is quite good at managing parents expectations (!) and has been known to refuse to write references in cases where its felt the child won't have a cat in hell's chance of getting into a particular school! Having said that, once "reasonable" school choices are made, the head really will "sell" the child on his /her reference and will also get on the phone to individual schools in borderline cases wherever possible.
Everyone is advised to sit a max of 4 schools (some more of a stretch than others) and NOT to tutor as this will be counterproductive (overload / confuse child).
There are 3 sets for Maths and for English. From about the summer half-term in Year 5, they all do a sample Maths paper in class every week (at an appropriate level). These are past papers from a range of schools. The same happens in English - comprehensions and short-story writing under timed conditions. They do some reasoning, but not much as this varies so much from school to school. They often mark their own papers and discuss as a class.
They were getting a timed story or comprehension every weekend with some timed maths as well, plus maths twice in the week -all geared towards the exams. So really there wasn't time for any tutoring!
They had a mock interview in school in the Autumn term of Year 6. They were told to just "be yourself, but on a good day", make eye contact, that kind of thing. They were not told whether to wear uniform or not on the day.
Of course the whole of Jan was fairly hideous, but they all got into their first choice schools and then basically went on trips, did plays and whatever else they fancied until the end of the year.
How much of an advantage they had is hard to gauge, however - nearly half the intake at dd's current school (Latymer Upper) were from state schools; some of these schools no doubt better than others. I don't think a good independent school can make children any "cleverer" - a bright child will be just that, regardless of where they're coming from to sit the exams. Maybe though the parents with children at independents are simply more guided through the process so there are less surprises?
Anyway, don't mock the blazers - when your dcs get all their offers they will soon be joining the ranks! Firm handshakes and good eye-contact all round!
(And its not all a bad thing -really)!