I have one daughter at CCHS and another starting in September.
I know girls that travel from Ilford and Romford so it can be done.Don't forget you also have Woodford County High within reach which is also , last I heard, a good school.
Westcliff and Southend have signiificantly lower marks needed to get in.As Essex Mum18 said, CCHS do not have a pass mark just take the top scorers.
I would say that you need , to be comfortable, to be scoring high eighties + in all papers.This is to allow for a drop on the day.My younger daughter got in the seventies and scraped in
but was doing much better at home practising.
Older one didn't drop marks on the day so it doesn't always happen.
Read the Essex part of the forum for more detail.