@Can you prove you're not a paedophile?
"a CRB check CAN prove someone IS / WAS ... does sexual preference change after a few years?"
Might I remind you of Operation Ore? Have you forgotten that people were brow-beaten into accepting Cautions for Child Porn offences which they had *NEVER* committed (they had just been the victim of credit card fraud) rather than have their names and reputations dragged through the Courts by Jim Gamble and co such that even if (when) they were acquitted, the taint of that charge would still linger?
Except, of course, now they have accepted that Caution, the CRB check *WILL* pick up on that and that is "proof" they are a paedophile, except, of course, they never were.
Meanwhile someone who wasn't caught by Gamble and Co but who has never been caught is obviously "safe" to work with children...