As Cheekychops says, the convictions will always be on file, and available for police and certain other agencies to see HOWEVER they will become spent after a certain period, which means that you then won't have to declare them on job applications (except for jobs working with children, elderly and vulnerable adults - there are some other exceptions - you should check the nacro website) The time until a conviction is 'spent' depends on the sentence recieved, again check the nacro website or google 'rehabilitation of offenders act 1974' Any offence receiving a sentence of 2.5 years imprisonment or more will never be spent.
http://www.nacro.org.uk/data/resources/nacro-2 005020105.pdf