Under the Rehabilitation of Offenders Act offences which were dealt with by way of a Conditional Discharge are deemed to be “spent” when the period of the Discharge is over (subject to a minimum period of one year).
However, some jobs are exempt from the requirements of the Act and it is also not applicable to the process of applying for visas to visit foreign countries. The US, for example, insists that all offences are declared, and even insists on details of police arrests that did not lead to criminal proceedings.
This leaflet from NACRO gives more details:
http://www.nacro.org.uk/data/resources/nacro-2007021302.pdf