If I were education secretary for the day, I'd get rid of faith admissions as my priority, but that's another issue.
I'd get rid of ALL forms of selection for state funded schools. The only way in which a comp system will work is if parents have to send their kids to a choice of 2/3 of the closest schools. Otherwise, you have selection by the back door, due to people moving or putting down false addresses, etc.
The faith schools have a far bigger impact of sucking away average/high achievers leaving everyone else in sink schools, than grammars or private schools, simply due to the far higher numbers of faith school pupils as opposed to private/grammar numbers.
In our city, there are two huge faith schools, each with 1,500-2,000 pupils, despite empty churches (lots of people lie to get their kids in them!). The comps don't stand a chance, we have three and they're all failing, none are bigger than 1,000 - we used to have four but the worse one got closed down and the kids are now bussed over the county border to a "good" comp.