Not too sure about England and Wales but in Scotland the Rehabilitation of Offenders Act cancels out the majority of offences after a given period of time. Once 'spent' these offences should not show up on a Disclosure Scotland certificate. Obviously the length of time before an offence is deemed to be spent depends on the severity of the offence commited.
Also, in Scotland if you are given a Police Caution/Warning for an Offence it does not have to be declared to an employer as it does not give you a 'Criminal Record' but this is not the case in England and Wales. I only know this as my brother (who is Scottish) was recently sacked for not declaring a caution for assault which occurred in London. He thought that, like Scotland, it did not give him a record in England.