As Baggybear said, you can not apply for even a basic CRB yourself, you have to be an employee.
Although these checks show a past history, (or lack of) they will, obviously, only show if anyone has been caught and charged doing XYZ, they do not show any pending cases that an individual has yet to be charged for.
You could have a squeaky clean past but be on trial for theft, murder, abusing trees lol whatever, this will not show up on your check as you've yet to be found guilty or charged.
So therefore you'd have a clean CRB check lasting 3 years and a week or a year later be charged with a string of offences but your CRB has already been done.
I personally don't think they're worth the paper they're written on, how many teachers and people of trust and authority have had them done and BOOM!! Up in court for God knows what