"No, it's nothing like that. Kent has all sorts of schools and the grammar schools are not all the same - far from it. They are highly differentiated from one another and operate in a market place. In Kent, state and private schools compete for the same DC and so you get specialities ie an all IB grammar school. In any case, lots of children in Kent do not take the Kent test (11+) - there is absolutely no obligation or expectation for DC to take it."
The grammar schools do not operate in a market place- they go by catchment.
It is very unusual for a clever child not to go in for the test- but obviously there is no compulsion to take it.
"Some Kent comprehensives have a grammar stream so DC who have taken and passed the Kent test go there."
This isn't true- there are no comprehensive schools in Kent, and it is very, very unusual for a child who passed the Kent Test to go anywhere but a grammar school. There are faith secondaries, but once again, they tend to be populated by children who didn't pass.
Some of the non selective schools have a grammar stream- but most do not have enough high achievers to make it a true grammar set. My ds is in th grammar set, and there are plenty of children in the set with him who scored low 4s in SATs.