A good friend of mine applied for a £10k a year prep in East Anglia at the end of Y2. It is considered to be an academically selective prep.
Her DS had to go in for half a day and just play, basically. My friend was just a tad smug about 2 little boys who a) cried and b) sat under a table and refused to come out.
Imagine her surprise when BOTH got in. Her DS, 4 years later, had to sit the entry exams for the upper school. The stress! The worry! But the ENTIRE year group of 50 passed (to her chagrin!- she feels at least 5 in her DS's class weren't made of the 'right stuff', but interestingly , ALL were the sons of the wealthiest parents! ).
Really, in today's world, what they want, unless the school is absolutely UNIQUE and REALLY fought over is:
a) your ability to pay the fees
b) for your child to not demonstrate psychopathic personality traits, and
c)not to have SEN unless this is part of the deal.
Turn up for your interview conservatively dressed and bob's your uncle.
I am not being glib- really! But for the vast majority of private schools, as long as your family fit into the spectrum of 'normal' AND you LOOK like you can keep paying, it won't be an issue.